.. ical 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. BImmunological 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.
CDeficiency 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. IVTHE FIGHT AGAINST DISEASE ANatural Defenses 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. BMedical Defenses 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. VPREVENTING DISEASE 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. VIHISTORY 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. AEpidemics 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 Hernn Corts first reached Mexico, the population of central Mexico had fallen from about 17 million to about 3 million. It is doubtful that Corts could have conquered the Aztecs as easily as he did had this disaster not befallen the Aztecs. BStigma 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.
VIILIFE 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. Robert Sikorski Richard Peters Human Disease. Microsoft Encarta Encyclopedia 2001. 1993-2000 Microsoft Corporation.
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