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 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