Human Disease Research Human Disease IINTRODUCTION 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), and the Vietnam War (1959-1975) combined. 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, include fever, inflammation, pain, fatigue, dizziness, nausea, and 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 immune deficiency 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.
IIINFECTIOUS 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, and parasitic worms. In addition, it has been theorized that some proteins called prions may cause infectious diseases. APathogens 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, and 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, and 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. See Parasite. 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. BSpread 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. CNew 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; and 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.
IIINONINFECTIOUS DISEASE 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 (see Occupational and Environmental 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. AHereditary 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 (see Genetic Disorders). 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 phys …