HUMAN DISEASE

Human Disease, in medicine, any harmful change that interferes with the normal appearance, structure, or function of the body or any of its parts. Since time immemorial, disease has played a role in the history of societies. It has affected—and been affected by—economic conditions, wars, and natural disasters. Indeed, the impact of disease can be far greater than better-known calamities.

An epidemic of influenza that swept the globe in 1918 killed between 20 million and 40 million people. Within a few months, more than 500,000 Americans died—more than were killed during

World War I (1914-1918)

World War II (1939-1945)

The Korean War (1950-1953)

The Vietnam War (1959-1975)

Diseases have diverse causes, which can be classified into two broad groups: infectious and noninfectious. Infectious diseases can spread from one person to another and are caused by microscopic organisms that invade the body.

Noninfectious diseases are not communicated from person to person and do not have, or are not known to involve, infectious agents.

Some diseases, such as the common cold, are acute, coming on suddenly and lasting for no more than a few weeks.

Other diseases, such as arthritis, are chronic, persisting for months or years, or recurring frequently.

Every disease has certain characteristic effects on the body.

Some of these effects, called symptoms and signs.

Fever

Inflammation

Pain

Fatigue

Dizziness

Nausea

Rashes

and are readily apparent to the patient.

These symptoms offer important clues that help physicians and other health care professionals make a diagnosis. Many times, however, the symptoms point to several possible disorders.

In those cases, doctors rely on:

Medical tests, such as blood examinations and X rays, to confirm the diagnosis.

The course of a disease—that is, the path it follows from onset to end—can vary tremendously, depending largely on the individual and the treatment he or she receives.

For example:

Otherwise healthy people usually recover quickly from a bout of pneumonia if given proper treatment, whereas pneumonia often proves fatal to people with a weakened immune system and to those who do not receive prompt, effective treatment.

Some diseases run a different course depending on the patient’s age.

Chicken pox, for instance, is usually mild in childhood but severe in adults.

In the United States, only about 5 percent of chicken pox cases occur in people over the age of 20, but these cases account for 50 percent of all deaths from the disease.

Scientists, public health officials, and other members of the medical community work diligently to try to prevent disease epidemics. The battle is constant and is fought on many fronts. There have been many victories. Once-devastating diseases such as smallpox and diphtheria have been virtually eradicated, and many other diseases that once conferred automatic death sentences can now be either cured or controlled.

At the same time, however, new killers have emerged.

Acquired immunodeficiency syndrome (AIDS) and hantavirus pulmonary syndrome are among at least 30 diseases that have been identified by scientists since the early 1970s.

Other growing challenges, particularly in the affluent societies of industrialized nations, are so-called diseases of choice, such as alcohol abuse, drug abuse, or obesity, that result from addictive behavior, poor eating habits, or insufficient exercise.

Complicating matters further are societal changes.

Increased international travel accelerates the spread of both new and old diseases: A person infected with an unusual virus on one continent can arrive—with the virus—on another continent in a matter of hours. Ships, planes, and trucks can transport disease-carrying organisms just as easily.

In 1985 tires imported into Texas from Asia carried larvae of the Asian tiger mosquito, which is a carrier of dengue fever and other tropical diseases.

Within five years, Asian tiger mosquitoes were living in 17 states.

Changing dietary habits and the availability in local supermarkets of foods from all parts of the world contribute to an increase in food-borne illnesses.

Some researchers worry that growing populations and the resulting crowded living conditions will increase the risk of epidemics.

INFECTIOUS DISEASE

Infectious diseases are caused by microscopic organisms commonly called germs.

Physicians refer to these disease-causing organisms as pathogens.

Pathogens that infect humans include a wide variety of bacteria,

viruses,

fungi,

protozoans,

parasitic worms.

In addition, it has been theorized that some proteins called prions may cause infectious diseases.

INFECTIOUS DISEASE

Pathogens

Bacteria are microscopic single-celled organisms at least 1 micron long. Most bacteria species are harmless to humans; indeed, many are beneficial (see eubacteria). But some are pathogens, including those that cause;

cholera

diphtheria

leprosy

plague

pneumonia

strep throat

tetanus

tuberculosis

typhoid fever

Viruses are tens or hundreds of times smaller than bacteria. They are not cellular, but consist of a core of genetic material surrounded by a protective coat of protein. Viruses are able to survive and reproduce only in the living cells of a host. Once a virus invades a living cell, it directs the cell to make new virus particles. These new viruses are released into the surrounding tissues, and seek out new cells to infect.

The roll call of human diseases caused by viruses includes;

mumps

measles

influenza

rabies

hepatitis

poliomyelitis

smallpox

AIDS

certain types of cancer

Fungi are a varied group of generally small organisms that get their food from living or dead organic matter. They germinate from reproductive cells called spores, which often have a thick, resistant outer coat that protects against unfavorable environmental conditions. This enables

spores to survive for long periods of time, which adds to the difficulty of treating fungal infections.

Some fungi are external parasites of humans, causing skin conditions such as ringworm, athlete's foot, and jock itch. Other fungi invade internal tissues; examples include yeast that infect the genital tract and several fungi species that cause a type of pneumonia.

Protozoans are single-celled, animal-like organisms that live in moist environments. Perhaps the most infamous pathogenic protozoans are species of the genus Plasmodium, which cause malaria, an infectious disease responsible for over 2 million deaths worldwide each year.

Members of the genus Trypanosoma produce trypanosomiasis, also known as African sleeping sickness, and Chagas' disease.

Other protozoans cause giardiasis, leishmaniasis, and toxoplasmosis.

Parasitic flatworms include tapeworms, which live in the intestines of a host organism. They have a ribbon-like body that may be up to 9 m (30 ft) in length, depending on the species. Hooks and suckers on the head attach a tapeworm to the intestinal wall, and a tough outer coating protects against the host's digestive juices.

Another group of parasitic flatworms is flukes, which are responsible for several serious tropical diseases, most notably schistosomiasis.

Roundworms, or nematodes, are small, tubelike worms that are pointed at both ends. Species that infect human intestines include pinworms, hookworms, threadworms, and members of the genus Ascaris.

Trichinella spiralis can invade human muscle tissue, often from eating infected pork that has been improperly prepared, causing a disease called trichinosis.
Prions are extremely tiny protein particles found in the brain, nerve, and muscle cells.

A controversial theory states that prions cause disease by changing normal proteins into an abnormal shape. These mutated proteins in turn force other proteins to change shape, leading to destruction of tissue, primarily in the brain.

Some researchers have hypothesized that prions cause transmissible spongiform encephalopathies, a group of rare infectious diseases that includes Creutzfeldt-Jakob disease in humans, scrapie in sheep, and bovine spongiform encephalopathy (commonly known as mad cow disease) in cattle.

Some evidence suggests that prion-related disease can be transmitted through food infected with mutated proteins.

INFECTIOUS DISEASE

Spread of Infectious Disease

Some pathogens are spread from one person to another by direct contact.

They leave the first person through body openings, mucous membranes, and skin wounds, and they enter the second person through similar channels.

For example, the viruses that cause respiratory diseases such as influenza and the common cold are spread in moisture droplets when an infected person coughs or sneezes.

A hand that was used to cover the mouth while coughing contains viruses that may be passed to doorknobs, so that the next person to touch the doorknob has a chance of picking up the infectious agent.

The bacteria that cause some sexually transmitted diseases, including gonorrhea and syphilis, are transmitted during sexual contact.

Other pathogens involve an intermediary carrier, such as an insect.

The malarial parasite, for example, spends part of its life cycle in mosquitoes, then enters a person's bloodstream when the mosquito bites the person.

Many pathogens are spread through contaminated food and water.

Cholera bacteria, for example, are spread through food and water contaminated with the excrement of infected people.

INFECTIOUS DISEASES

New Infectious Diseases

In 1978 the United Nations adopted a resolution that set goals for eradicating infectious disease by the year 2000. This lofty goal proved impossible to achieve. The years since the resolution was adopted have seen the emergence of new killers and a rise in the incidence of such ancient scourges as malaria, yellow fever, and tuberculosis.

Among the diseases new to science are AIDS, Ebola hemorrhagic fever, Legionnaires’ disease, and Lyme disease.

AIDS has been the most deadly of all the new diseases, but even it has not taken as high a toll as malaria, tuberculosis, and other diseases that have been around for centuries.

Some newly identified disease-causing agents for diseases that have been recognized for a long time include

Human T-lymphotropic virus I (HTLV-1), which can cause some cases of non-Hodgkin’s lymphoma, a type of cancer originating in the lymphatic system.

HTLV-2, which is associated with hairy-cell leukemia, a rare type of cancer of the blood.

In most cases, the reasons for the emergence of a new disease are unknown. One exception is Legionnaires’ disease. It is caused by a bacterium that was not identified until after an outbreak in 1976 at an American Legion convention in Philadelphia, Pennsylvania.

Once identified, however, scientists were able to retrospectively identify earlier epidemics of the disease, and realized that each year the bacterium is responsible for thousands of cases of pneumonia.

Environmental changes may be responsible for some new diseases. Scientists speculate that the viruses for some of the deadly hemorrhagic fevers that have surfaced in Africa, such as Ebola and Marburg disease, have long existed in certain wild animals.

As people have encroached on wilderness areas they have come into contact with the infected animals, and the viruses have jumped from their traditional animal host to a new human host, with deadly consequences.

In addition to new diseases, well-known pathogens may change, or mutate, creating new, virulent strains.

Influenza viruses are among those that mutate frequently, which explains why flu shots—vaccines that use modified or killed versions of the influenza agent to stimulate a protective immune response in the body—are given annually, and why epidemics of influenza periodically occur.

The strains of flu virus that were most prevalent one year differ from those that bedevil humans the next year. Vaccines that protected against last year's flu virus may need to be altered to be effective against today's most common strains.

A similar problem occurs when mutations in infectious agents result in resistance to medicines that had been effective treatments.

The bacteria that cause bronchitis, meningitis, tuberculosis, and pneumonia are among many that have developed strains that are resistant to at least some antibiotics. As a result, doctors have fewer options for treating the diseases and preventing their spread.

NONINFECTIOUS DISEASES

Diseases not known to be caused by infectious agents include the three leading killers in the United States and other developed countries: heart disease, most cancers, and cerebrovascular disease (decreased blood circulation in the brain). Noninfectious illnesses include disorders as terrifying as Alzheimer's disease, which robs victims of their memory and their ability to reason, and as pesky as poison ivy.

Degenerative disorders, including arthritis, Parkinson disease, and Alzheimer's disease, involve the progressive breakdown of tissues and loss of function of parts of the body. Joints gradually become stiff; bones become brittle; blood vessels become blocked by deposits of fat. The incidence of these problems increases with age (see Aging), and, in at least some cases, progression can be slowed by good health habits.

Environmental factors play critical roles in numerous noninfectious diseases. Exposure to carbon monoxide can have long-term effects on the heart and vision. Lead in drinking water can impair children's mental abilities and increase blood pressure in adults.

Occupational exposure to ;

coal dust,

cotton dust, and

asbestos predisposes workers to black lung, brown lung, asbestosis, and other respiratory diseases.

Other diseases are caused by an addiction to a harmful substance.

Tobacco smoking is a prime culprit in emphysema, as well as lung cancer and other respiratory diseases. Excessive use of alcohol can lead to liver disease, brain damage, and nutritional disorders.

Repetitive stress injuries result from repeating certain motions, usually from a fixed or awkward posture. Twisting items on a factory assembly line, carrying bulging mailbags, using vibrating tools such as pneumatic hammers, or practicing the piano or a tennis stroke for hours on end can all result in pain, inflammation, and permanent nerve damage.

NONINFECTIOUS DISEASES

Hereditary and congenital Diseases

Hereditary diseases such as hemophilia, sickle-cell anemia, Huntington's disease, muscular dystrophy, and Tay-Sachs disease are caused by mutated genes inherited from one or both parents .

Certain other diseases, such as diabetes mellitus, hypertension, and some types of cancer, often run in families, which suggests that heredity is at least partially responsible for their development.
Congenital diseases, or birth defects, are disorders that are present at birth. Some are hereditary, others develop while a baby is in its mother's uterus or during the process of delivery.

For example, if the mother contracts German measles, or rubella, during the early stages of pregnancy, her child may be born with heart defects, eye cataracts, deafness, or mental retardation.

Use of alcohol during pregnancy can cause fetal alcohol syndrome, characterized by mental and physical retardation.

Abnormal development of any body part in a fetus may produce a congenital defect; for example, if walls that separate the chambers of the heart fail to form completely, the baby is born with congenital heart disease.

NONINFECTIOUS DISEASE

Immunological Diseases

Immunological diseases occur when the immune system, which normally protects against infections, malfunctions. The most common types of immunological diseases are allergies, autoimmune diseases, and immune deficiencies.
An allergy is an abnormal reaction of the immune system to foreign substances, such as plant pollen, fungal spores, animal danders, medications, and foods. Rhus dermatitis is an allergy caused by contact with urushiol, an oil resin produced by poison ivy, poison oak, and poison sumac.

Autoimmune diseases develop when the immune system goes awry and attacks the body's own tissues. Autoimmune disorders includes lupus erythematosus, rheumatoid arthritis, juvenile-onset diabetes, and myasthenia gravis. The causes are unknown, although some scientists suspect the diseases may be triggered in some cases by a pathogen, such as a virus, or other environmental factor.

Immune deficiency diseases develop when the immune system becomes impaired, resulting in more common, frequent, or severe infections. The immune system may be damaged by a genetic abnormality or by illness, injury, the use of a strong drug such as those used in chemotherapy, or malnutrition.

NONINFECTIOUS DISEASE

Depiciency diseases

Deficiency diseases result from insufficient amounts of various healthful nutrients in the diet. Examples include scurvy, caused by a deficiency of vitamin C, or ascorbic acid; pellagra, caused by a deficiency of niacin; and osteoporosis, caused at least in part by a lack of calcium. Deficiency diseases are most prevalent in poverty- or war-stricken areas of the world, where malnutrition is widespread. Deficiency diseases are also found in more affluent nations where food is prevalent but people’s food choices or behavior do not provide well-rounded nutrition, resulting in such disorders as anorexia nervosa, bulimia, and anemia.

FIGHT AGAINST DISEASE

Natural Depenses

The skin and mucous membranes form the body's first line of defense against disease. Most microscopic pathogens, or microbes, cannot pass through unbroken skin, although they can easily enter through cuts and other wounds. Mucous membranes protect internal organs that are connected with the outside of the body. These membranes, which line the respiratory, digestive, urinary, and reproductive tracts, secrete a sticky fluid called mucus, which traps microbes. The mucus may then be expelled from the body, perhaps in a cough or sneeze or in feces. If the mucus is swallowed, digestive juices kill the microbes.

Small hairlike projections on the lining of the nose, throat, and bronchial tubes work in conjunction with mucus to trap and remove foreign substances. In the ears, tiny hairs plus a sticky wax defend against the entry of germs. Tears secreted by the lachrymal gland wash away germs and other small objects that may enter the lid area of the eye. Tears also contain a protein that kills certain germs.

If a pathogen breaches the body’s outer barriers, the defenses of the immune system spring into action. Some of these defenses are effective against a variety of invaders, while others are tailor-made to fight a specific organism. White blood cells called phagocytes constantly travel through the bloodstream on the lookout for foreign objects. If they come upon a microorganism, they surround, engulf, and digest it.

If the infection persists and there are too many organisms for the phagocytes to fight by themselves, the immune system produces proteins called antibodies. Each antibody is designed to combat a particular antigen, or foreign protein. Two types of white blood cells are involved in this process. B cells release the antibody, which attaches to the outer covering of the antigen, marking it for destruction. T cells attack the tagged antigen and also stimulate B cells into action. Once the body has produced antibodies to a specific microorganism, it generally is immune to future invasions by that organism. That is why people who have had chicken pox or measles as a child will not get the disease again as an adult. The reason people get one cold after another is that each cold is caused by a different virus strain.

THE FIGHT AGAINST DISEASE

Medical Depenses

Much of early medicine was practiced by trial and error, but ancient peoples also looked for causes and cures for disease by studying the body and observing the sick.

In Greece during the 5th century bc, the physician Hippocrates stressed that medical care was a science that could be learned through clinical observation and experimentation.

The connection between health and hygiene was made in several ancient cultures, including those of India and Rome. The Romans drained marshes where malaria-carrying mosquitoes bred, and they built underground sewers and aqueducts to carry clean water in the cities.

Laws governed the cleanliness of streets and the storage of food.

Because of limited contact between cultures, most early knowledge of the efficacy of various measures did not spread from place to place. With the collapse of the Roman Empire around ad 400, much medical knowledge was lost, to be replaced by superstition. It was not until the 14th century that a medical renaissance began.

Thereafter, progress occurred exponentially. Accurate descriptions of the structure and functioning of the human body were made, and the invention of the printing press in the middle of the 15th century enabled this information to be published and easily disseminated.

The development of microscopes in the late 16th century prompted the discovery of microorganisms, although it was not until the 19th century that scientists were able to show that bacteria and other microbes caused disease. Also in the 19th century, people recognized the importance of sanitation and cleanliness, improving the survival rate in hospitals.

Anesthesia was discovered and the first vaccines were produced.

During the 20th century, the importance of vitamins and other nutrients in preventing disease was recognized. Antibiotics, sulfa drugs, blood types, and genes that cause disease were discovered. A host of diagnostic and surgical tools were created that incorporated inventions such as X rays, fiber optics, lasers, and computers. Techniques such as organ transplantation (see Medical Transplantation), kidney dialysis, dental implants, gene therapy, and fetal surgeries were introduced. Thousands of new drugs were developed to treat everything from ulcers to zinc malabsorption.

The list of medical techniques for fighting disease continues to grow. More effective methods are expected to be introduced in the coming years as scientists gain a better understanding of such subjects as the molecular biology of normal and abnormal cells, gene structure and action, and the relationship between environmental stresses and disease.

PREVENTING DISEASES

It is much less costly, in terms of both human suffering and economics, to prevent disease than to treat it. Public health services and medical professionals play critical roles in helping people avoid disease. In addition, each individual plays a vital role in protecting his or her personal health.
Public health services are charged with protecting community health. Their activities include provision of adequate clean water and the sanitary disposal of sewage and other wastes. Food supplies—on farms, at food processing plants, and in supermarkets and restaurants—are inspected for microorganisms. Pesticide spraying programs are undertaken to control populations of mosquitoes and other carriers of disease. Public facilities, such as schools and hospitals, are inspected to ensure that they meet appropriate standards of cleanliness and safety. Education and surveillance programs alert physicians and other medical workers to disease threats.
Physicians, dentists, and other medical experts have a number of preventive tools at their disposal. Among the most effective are vaccines, which stimulate the immune system to produce antibodies against particular antigens. A vaccine may contain killed or weakened pathogens, parts of the pathogens, or modified toxins produced by the pathogens, which are strong enough to arouse the immune system to fight off new invading pathogens but not powerful enough to cause disease themselves. Thanks to vaccines, polio is rare today, smallpox has been eliminated, and diseases such as diphtheria and whooping cough, which once killed many young children, have largely been brought under control.
Regular medical check-ups are another important preventive tool. These help doctors to find disease in its early stages, when it is easier to treat and before it causes significant damage. For example, during a check-up a dentist will remove plaque, a sticky bacterial coating on teeth. Left undisturbed in hard-to-reach areas, such as between the teeth and along the gums, plaque can lead to periodontal disease, which can destroy the tissues that anchor the teeth in the mouth (see Dentistry).
Even the finest public health and medical services are of limited value to people who have poor health habits. Numerous studies have proven that physical health and longevity are linked to the following: eating a balanced diet, maintaining proper weight, exercising regularly, using condoms and limiting the number of sexual partners, avoiding tobacco, and avoiding alcohol or consuming it in moderation. People who fail to follow these guidelines increase their risk of cardiovascular disease, cancer, AIDS, hepatitis, and other lethal diseases.
The interplay among public health measures, medical practices, and personal responsibility is exemplified in the fight against tooth decay. Caused by bacteria that feed on food debris in the mouth, tooth decay can be virtually eliminated through a combination of three steps: the addition of fluoride to public drinking water supplies; the professional coating of teeth with a plastic sealant, which fills microscopic pits where bacteria can collect and cause decay; and regular brushing and flossing of teeth.

HISTORY OF HUMAN DISEASE

Humans have always had to deal with disease.

Skeletons more than 12,000 years old show evidence of tuberculosis and other diseases. The 9400-year-old mummified remains of Spirit Cave man, found in Nevada in 1940, indicate that he suffered from back problems and tooth abscesses.

The remains of Ramses V, ruler of Egypt around 1150 bc, show that his face was disfigured by smallpox scars.

Disease has had a dramatic impact on human history. For most of the 250,000 years that humans have been on the earth, disease has played a central role in limiting population growth. As ways to combat disease were discovered, people lived longer and had more children, who lived long enough to have children of their own.

The human population slowly increased and then exploded. By 1804 the human population reached 1 billion. Just over 100 years later, in 1927, after the advent of the first vaccines and the recognition of the importance of sanitation and safe water supplies, the population had doubled to 2 billion. By 1974 it had doubled again to 4 billion.

Since then, recognition that the earth's environment has a limited capacity to support an ever-increasing population has led to concerted efforts to limit population growth. Nevertheless, as the 20th century neared its end, the population had reached 6 billion. It is expected to rise to more than 8 billion by 2021.

HISTORY OF HUMAN DISEASE

Epidemics

Periodically, devastating outbreaks of infectious disease occur, affecting many people in a region at the same time. Such outbreaks are called epidemics. Those of widespread proportions, such as the current AIDS epidemic, are often referred to as pandemics.

People have always been fearful of epidemics and their effects. In China in the 13th century bc, the ruler of Anyang asked his diviners, "Will this year have pestilence and will it be deaths?"

In Egypt around 2000 bc, a writer compared fear of the Pharaoh with fear of epidemics. The Old Testament of the Bible refers to several epidemics, including one that affected the Philistines, purportedly as punishment for seizing the Ark of the Covenant. The British Isles were hit by at least 49 epidemics between ad 526 and 1087.

Epidemics can reshape societies, affect the course of military events, and change the balance of power among different groups of people. An epidemic in Athens in 430 bc created chaos in the city and contributed to defeat in its war with Sparta.

Among the best known of all epidemics was the Black Death, an epidemic of bubonic plague that broke out in Europe in ad 1347. By 1351 an estimated 25 to 50 percent of the people in Europe had died from the disease. The Black Death depopulated once-flourishing cities, left villages vacant, and caused a decline in cultivated land.

When Europeans began to explore the Americas in the 15th century, they carried along pathogens unknown in the new lands. Smallpox and measles raced through native populations with devastating results.

For example, by 1568, only 50 years after Hernán Cortés first reached Mexico, the population of central Mexico had fallen from about 17 million to about 3 million. It is doubtful that Cortés could have conquered the Aztecs as easily as he did had this disaster not befallen the Aztecs.

HISTORY OF HUMAN DISEASE

Stigma of Disease
Fears of disease, often coupled with ignorance, have led to horrifying treatment of the afflicted. Outbreaks of plague in Europe were often blamed on Jews, who were beaten and driven from their homes.

During an epidemic in 17th-century Italy, people suspected of being carriers of the plague were tortured and burned alive. Through the ages people with leprosy were often isolated in leper houses, forbidden to marry, and forced to wear a distinctive cloak or shake a rattle to announce their presence.

Even in supposedly advanced cultures, the stigma of disease remains.

In recent years, people with AIDS have heard that their illness was God's punishment for immoral behavior. Many have been ostracized by family, friends, and even physicians who are fearful of contagion.

People with AIDS have also been denied housing, medical treatment, and the right to travel to foreign countries.

LIFE SPAN

At the beginning of the 20th century, people in the United States had an average life span of about 50 years. By the time the century neared its close, average life span had risen to 76 years. Other developed countries experienced similar increases. Much of the credit for these longer life spans—and for the good health that accompanies them—is due to the conquering of diseases, thanks to vaccines, antibiotics, sophisticated surgical tools, and other medical miracles. The challenges ahead include bringing the benefits of this medical knowledge to all peoples of the world, and expanding on current knowledge in order to understand, treat, and prevent the diseases that still confront us.

PUBLIC HEALTH SERVICES

Public Health Service, agency of the U.S. Department of Health and Human Services, established in July 1798 under the terms of congressional legislation authorizing marine hospitals for seamen in the merchant marine. Subsequent legislation vastly increased the scope of the agency's activities. Today it is the principal health agency of the federal government. The Public Health Service is charged with promoting the highest level of health attainable for every American and cooperating with foreign governments in health projects. It provides financial assistance for the development and delivery of local health services, for education in the health professions, and for research in medical sciences. Among its other functions are protecting the population against unsafe food and drugs and preventing or controlling communicable diseases.
The service, administered by an assistant secretary for health, includes the following major units: Agency for Toxic Substances and Disease Registry; Alcohol, Drug Abuse, and Mental Health Administration; Centers for Disease Control; Food and Drug Administration; Health Resources and Services Administration; Indian Health Service; and the National Institutes of Health.

PUBLIC HEALTH SYSTEMS

Public health systems vary in different parts of the world, depending upon the prevalent health problems. In the developing world, where sanitation problems and limited medical resources persist, infectious diseases are the most significant threat to public health. Public health officials devote resources to establish sanitation systems and immunization programs to curb the spread of infectious diseases, and provide routine medical care to rural and isolated populations. In industrialized nations, sanitary food and water supplies and excellent medical resources have reduced rates of infectious disease. Instead, accidents and diseases such as lung cancer, heart attacks, and strokes are among the leading causes of death. In these areas, public health goals include education programs to teach people how to prevent accidents and lessen their risk for disease, and the maintenance of the excellent disease prevention systems already established.
Public health workers may engage in activities outside the scope of ordinary medical practice. These include inspecting and licensing restaurants; conducting rodent and insect control programs; and checking the safety of housing, water, and food supplies. In assuring overall community health, public health officials also act as advocates for laws and regulations—such as drug licensing or product labeling requirements. Some public health officials are epidemiologists, who use sophisticated computer and mathematical models to track the incidence of communicable diseases and to identify new diseases and health trends. Others conduct state-of-the-art medical research to find new prevention and treatment methods.
Most people think of public health workers as physicians and nurses, but a wide variety of other professionals work in public health, including veterinarians, sanitary engineers, microbiologists, laboratory technicians, statisticians, economists, administrators, attorneys, industrial safety and hygiene specialists, psychologists, sociologists, and educators.

PUBLIC HEALTH PROGRAMS

Immunization

One of public health’s greatest success stories, immunization is one of the most effective weapons available to combat the spread of infectious disease.

Immunization is the process of making the body resistant to a specific disease by using a vaccine, a chemical that stimulates the body to create antibodies to fight a specific infectious organism. In industrialized nations, vaccination programs protect children against measles, mumps, diphtheria, and other childhood infectious diseases.

In the United States, public health agencies provide these immunizations free of charge to children from low-income families. When small outbreaks of infectious disease threaten to grow into epidemics, public health officials may initiate new vaccination programs.

For example, in the late 1980s outbreaks of measles erupted in young adults who had been immunized once as infants. Public health officials recognized that these people may have lost their immunity and established a new vaccination program requiring a measles vaccination at 15 months and also at 4 to 6 years of age to boost immunity.

Several infectious diseases have been virtually eradicated by immunization programs. By 1979 a worldwide vaccination program had eliminated smallpox, a viral disease once responsible for more than 2 million deaths a year.

Poliomyelitis, commonly known as polio, has been virtually eliminated from most developed nations of the world, and the incidence of tetanus, whooping cough, and diphtheria has been drastically reduced worldwide.

Global vaccination programs require extensive financial support and an army of health care professionals over long periods of time. Prohibitive costs prevent millions of children in developing countries from being immunized against measles, mumps, and other easily preventable childhood diseases.

Even when resources are available, poor road and distribution systems and regional political upheavals may limit the success of vaccination programs.

For example, immunization efforts to eradicate Guinea worm, a parasite that causes painful and often debilitating skin infections, have reduced annual incidence from 1 million cases in 1981 to 80,000 cases in 1997. But momentum toward complete eradication has slowed significantly because the remaining cases are primarily in western and central Africa, especially in the Republic of the Sudan, where civil war interferes with national and international public health efforts to eliminate the disease.

The United Nations estimates that up to 4 million lives could be saved annually if existing, but under-used vaccinations were fully implemented.

PUBLIC HEALTH PROGRAMS

Rural and Urban Health Clinics

Public health agencies operate local clinics that provide free or reduced-cost medical services to individuals, especially infants and children, pregnant and nursing women, migrant farm workers, and people with drug abuse problems, physical disabilities, and other conditions.

These clinics provide prenatal and pediatric care for children who have no regular access to medical care. The clinics may provide visiting nurse and other home health care services for the elderly. Public health clinics may also offer rehabilitation programs for people addicted to drugs or alcohol.

In developing countries and in rural and low-income communities in industrialized nations, public health clinics may be the only source of routine medical care. Members of the community go to clinics for regular exams or for treatment when they are injured or ill, as well as for immunizations and other public health services.

Clinics may provide free or low-cost medications, such as antibiotics or birth control pills. Often, community members may obtain counseling on health matters, such as proper infant nutrition, safe food handling procedures, or family planning advice.

Public health clinics routinely screen patients for a number of infectious diseases, such as sexually transmitted diseases, and may provide free treatment if patients test positive.

Each clinic tracks the incidence of certain communicable diseases in its area, and reports this information to national and international public health offices.

Public health clinics may also track down past sexual partners of STD patients, inform them that they may have been infected with an STD, and urge them to come in to a clinic to be tested.

PUBLIC HEALTH PROGRAMS

Disease Tracking and Epidemiology

One branch of public health, epidemiology, studies the incidence of disease in large populations. Epidemiologists study data from public health clinics and private physicians who are required by law to report cases of certain diseases and deaths to public health officials. Using clues about the patterns in which deaths and diseases occur, epidemiologists are able to identify emerging disease patterns that may indicate environmental health hazards, potential outbreaks of existing diseases, or emergence of new infectious diseases.

For example, in the early 1980s a few doctors in New York and California reported an unusually high number of deaths from opportunistic infections—infections by organisms that usually only cause disease in people with weakened immune systems.

These early reports prompted epidemiologists and medical researchers to investigate this new vulnerability to opportunistic infections, which they called acquired immunodeficiency syndrome (AIDS). These efforts led to the identification of the previously unknown human immunodeficiency virus (HIV), the virus that causes AIDS.

Threats to public health concerns change over time and epidemiologists and other officials continuously evaluate epidemiological trends to determine how best to meet future public health needs.

For example, recent epidemiological reports show that tuberculosis, an infectious disease believed to be under control just 30 years ago, is now responsible for more deaths worldwide than any other infectious disease, killing more people per year than AIDS and malaria combined.

This resurgence is due to new drug-resistant strains of the bacteria that cause tuberculosis. The tuberculosis epidemic, or pandemic, has been declared a global public health emergency, prompting intensive international public health efforts to curb its spread.

Epidemiologists and other public health officials attempt to break the chain of disease transmission by notifying people who may be at risk for contracting an infectious disease.

When epidemiologists learn that a restaurant worker has infectious hepatitis, they place announcements in local media, such as radio and newspapers, urging people who ate at the restaurant in recent weeks to be checked for the disease and to seek treatment so that they will no longer risk infecting others.

Organizations such as the World Health Organization (WHO), an agency of the United Nations, monitor infectious diseases and try to prevent outbreaks from spreading worldwide.

In early 2003 WHO epidemiologists learned that a deadly type of pneumonia of unknown cause that originated in mainland China was quickly spreading to other countries.

The disease, known as severe acute respiratory syndrome (SARS), begins with a fever, chills, headache, and malaise. Two to seven days later some people develop a dry cough and difficulty breathing and, in some cases, die.

In March 2003 WHO issued an international alert, designating SARS as a worldwide health threat. The alert triggered public health officials from nations around the world to establish procedures to identify and isolate SARS cases and possibly prevent the illness from reaching epidemic proportions.

PUBLIC HEALTH PROGRAMS

Sanitation and Pollution Control
Disease-causing organisms are often transmitted through contaminated drinking water. The single most effective way to limit water-borne diseases is to ensure that drinking water is clean and not contaminated by sewage.

In many parts of the world, public health officials establish sewage disposal and solid waste disposal systems, and regularly test water supplies to ensure they are safe. These systems are expensive to develop and maintain, however, and many developing nations have poor or insufficient water sanitation systems.

The United Nations estimates that between 5 and 10 million people die each year from water-borne diseases such as cholera.

Photo Researchers, Inc./Sinclair Stammers Many diseases, such as hepatitis A and those caused by the Salmonella bacteria, are transmitted through food. When food is not washed or thoroughly cooked, or when food is stored at temperatures that are hospitable to disease-causing organisms, people who eat the food are subject to infection.

Public health programs establish and enforce laws for safe food storage and preparation. For example, in most nations food-processing plants, restaurants, and grocery stores are legally required to follow strict food-safety guidelines established by public health officials.

Phototake NYC/Jan Callagan Public health officials also establish and oversee programs to control flies, rats, and other animals that spread disease-causing microbes.

For example, pesticide programs in parts of Africa significantly reduced rates of trypanosomiasis, a sometimes fatal disease commonly known as sleeping sickness that is transmitted by the African tsetse fly. Several often deadly diseases are transmitted by rodents.

Hantaviruses transmitted by mice, for example, can cause a deadly type of severe respiratory disease. Humans are infected when they inhale dust containing virus-infected rodent feces. By preventing rodents from living in or near human dwellings, public health officials seek to reduce rates of these types of infections.

Environmental pollution is another preventable cause of disease and disability, and in most countries public health officials address the adverse health effects of air pollution and water pollution.

Public health officials may work in conjunction with pollution control organizations to establish and enforce pollution limits and advise the general population when pollution levels exceed safe limits.

For example, when air pollution reaches harmful levels in Los Angeles, California, public health and environmental officials warn the public of the potential dangers. When chemical contamination renders ponds or lakes unsafe for swimming, health officials post warning signs informing the public of the hazard.

They may close down businesses that consistently put harmful pollutants into the environment, or prohibit distribution and sale of food harvested from contaminated regions.

PUBLIC HEALTH PROGRAMS

Medical Research

Another component of public health is scientific and medical research. A cadre of doctors and scientists work in laboratories around the world to establish new ways to prevent, diagnose, treat, and cure disease and disability.

For example, over 30,000 different biomedical research projects are underway in the United States to investigate diseases such as AIDS, cancer, Alzheimer’s disease, and even the common cold.

Other projects investigate the safety and effectiveness of existing pharmaceuticals and treatment programs, and test the safety of hundreds of the products that we use everyday, such as new food products, household cleaners, and nonpolluting forms of gasoline.

Scientists and doctors employed by the government conduct some biomedical research in public health facilities. In other cases, biomedical research projects beneficial to public health, but conducted by nongovernmental scientists, receive public health funding to help cover the expenses associated with the research.

These research projects may be conducted by university professors, doctors and scientists at hospitals, and even private biomedical research firms, all working to find better ways to protect human health.

PUBLIC HEALTH PROGRAM

Public Education Campaigns

Many diseases are preventable through healthy living, and a primary public health goal is to educate the general public about how to prevent noninfectious diseases.

Public health campaigns teach people about the value of avoiding smoking, getting treatment for high blood pressure, avoiding foods high in cholesterol and fat, and maintaining a healthy body weight.

Other campaigns educate the public on ways to prevent birth defects, such as abstaining from alcohol during pregnancy to prevent fetal alcohol syndrome.

A broad area of public health education, called health promotion, places special emphasis on illness and disabilities that decrease the quality of life.

Health promotion efforts in the 1990s, for instance, increased public awareness of osteoporosis, a condition that causes disabling bone fractures among older women.

Programs educated women about how to maintain strong bones by including enough calcium in their diets, exercising, and to consider estrogen replacement therapy after menopause.

Health promotion also encourages people to take advantage of early diagnostic tests that can make the outcome of disease more favorable. Regular mammograms encourage early detection of breast cancer, for instance, increasing the chances of a cure.

Detection and proper treatment of high blood pressure reduces the risk of a stroke, the leading cause of permanent disability in older people.

Accidents, particularly automobile accidents, pose a major threat to public health, and officials have undertaken campaigns to reduce the number of automobile accidents by encouraging seat belt use and discouraging drinking and driving.

Violence, especially domestic violence and handgun accidents, is often regarded as a public health problem and public health departments sponsor family violence prevention programs and handgun safety programs.

PUBLIC HEALTH AGENCIES

On an international scale, public health is overseen by the World Health Organization (WHO). WHO manages international disease prevention and control efforts, and is involved in training medical personnel, educating world populations about public health issues including widespread diseases, nutrition, population control, and the benefits of environmental sanitation. WHO and all of its programs are funded by United Nations membership dues and contributions, and voluntary donations from public and private sources.
In the United States, the Department of Health and Human Services (HHS) oversees public health and welfare throughout the nation. It is funded by American citizens through taxes. The Public Health Service, which is part of HHS, is the main federal agency concerned with public health. The Public Health Service is divided into several individual agencies that specialize in different aspects of public health. The National Institutes of Health, the medical research arm of the Public Health Service, conducts biomedical research and maintains 25 institutes and centers of health and the National Library of Medicine. The Centers for Disease Control and Prevention (CDC) monitors and prevents outbreaks of diseases, maintains national health statistics, and administers many of the public immunization programs in the United States. The Food and Drug Administration (FDA) assures the safety of food, pharmaceuticals, cosmetics, and medical devices through legislation and licensing programs.
Each state in the United States has its own department of health and most local communities have a city or county health department as well. State and regional public health departments work with national agencies to administer public health programs. Other government agencies, such as environmental protection departments, also may have some responsibilities for public health. Hundreds of private or voluntary health organizations contribute to overall community health by educating the public about good health practices and prevention of heart disease, cancer, and other conditions.
Although most industrialized countries follow this general pattern for administration of public health services, there are individual variations. In the United States, for instance, social welfare programs, such as financial aid to poor families, are separate from public health, while in the United Kingdom, health and welfare services are combined. Differences also exist in public health financing. The United Kingdom, Canada, Switzerland, Japan, and most other industrialized countries have national health systems that provide free individualized medical care as well as public health services. In the United States, most public health services are provided by the government without charge, but individual medical care is primarily paid for by individuals or by health insurance.
Developing countries usually have small, poorly financed public health services. In many of the world's poorest countries, especially in sub-Saharan Africa, there is less distinction between public health and routine individual medical care. A national ministry of health usually provides all health services through a system of regional health centers. Regional medical officers administer medical programs in each region, overseeing populations of about 100,000 people. Health centers, staffed by nonphysicians with medical training, often serve populations of 10,000 to 20,000 people dispersed throughout many villages. Some villages have health outposts called dispensaries, where villagers can go for medicine, vaccinations, and other basic medical services. But many medical services readily available in the industrialized world, such as high-tech diagnostic tests or expensive medicines, are not available in developing countries.
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