DISEASE

a harmful deviation from the normal structural or functional state of an organism. A diseased organism commonly exhibits signs or symptoms indicative of its abnormal state. Thus, the normal condition of an organism must be understood in order to recognize the hallmarks of disease. Nevertheless, a sharp demarcation between disease and health is not always apparent. The study of disease is called pathology. It involves the determination of the cause (etiology) of the disease, the understanding of the mechanisms of its development (pathogenesis), the structural changes associated with the disease process (morphological changes), and the functional consequences of these changes. Correctly identifying the cause of a disease is necessary to identifying the proper course of treatment. Humans, animals, and plants are all susceptible to diseases of some sort. However, that which disrupts the normal functioning of one type of organism may have no effect on the other types. an impairment of the normal state of an organism that interrupts or modifies its vital functions. A brief discussion of disease follows. The subject is treated in a variety of articles. For a general discussion of human, animal, and plant diseases, see disease. For a discussion of diseases categorized according to their cause or transmission, see infection; nutritional disease; occupational disease. For a discussion of diseases associated with particular stages of human development, see childhood disease and disorder; growth; development. For a discussion of malignancy, which may affect any organ or tissue in the body, see cancer. For a discussion of disease-causing organisms such as viruses, bacteria, and parasites, see bacteria; virus. For a discussion of bodily defenses against disease, see immune system. For a discussion of the diagnosis and treatment of disease, see diagnosis; therapeutics; drug; medicine, history of. For a discussion of diseases affecting particular organs, tissues, or processes, see blood disease; cardiovascular disease; digestive system disease; endocrine system, human; renal system disease; skin disease; metabolic disease; muscle disease; nervous system disease; reproductive system disease; respiratory disease; eye disease and ear disease; connective tissue disease. For a discussion of neuroses and psychoses, see mental disorder. For a discussion of alcoholism and other drug addictions, see alcohol consumption. Disease most commonly is caused by the invasion of an organism by one or more outside agents. Typically the infectious organisms are microorganisms (e.g., bacteria, viruses, and fungi), but they also can include larger organisms such as parasitic worms or nonliving but harmful substances such as toxins or ionizing radiation. Disease also may result from changes within the organisman anatomical fault (congenital or acquired) or a physiological malfunction (e.g., diabetes mellitus, in which the body fails to secrete or adequately utilize insulin, a hormone that regulates blood-sugar levels). Other diseases are a combination of external and internal factors. An organism's failure to adapt to changes in its environment can produce damaging changes within it. Physiological malfunctions and disturbances of normal growth can be induced by changes of diet or by invasion of microorganisms or other agents. Nearly all organisms are able to defend themselves against most diseases. Humans and other vertebrates have developed two strategies of resistance, called immunity, to invading agents: nonspecific immunity, which is present in all vertebrates at birth; and specific immunity, which is acquired only after stimulation by the presence of a certain microbe or its products (e.g., the virus that causes chicken pox). Immunity also can be stimulated artificially in humans or other animals by inoculating them with microorganisms that have been killed (as in typhoid vaccine) or weakened (attenuated) ones (as in measles vaccine), which produce the defensive immune reaction without causing the disease. Sometimes an organism's defensive reaction to invasion by an outside agent can become part of the disease. The crippling of the lungs produced by tuberculosis is caused partly by the destruction of lung tissue by the invading microorganism (in humans, usually Mycobacterium tuberculosis) and partly by the fibrous tissue that the body lays around the infection in a defensive reaction. Disorders of the immune response itself can produce autoimmune disease (e.g., rheumatoid arthritis) in which the immune response is triggered not by an outside invader but by the body's own tissues, which some cells fight against and try to reject. The immune system also can be disabled by an invading microorganism, as is the case with the disease AIDS. Not all organisms that invade another produce disease. Some can establish a mutually beneficial relationship with their host without impairing its vital systems; for example, the bacteria that live in the gastrointestinal tracts of humans and other vertebrates make possible the digestive processes of their hosts. In addition, organisms that are pathogenic to one species may be harmless to another. A disease that becomes established in an organism usually requires some form of treatment. In most cases, treatment consists of administering drugs that kill the causative agent, restore any physiological or biochemical imbalances that have occurred, or control the symptoms caused by the agent so that the affected organism can continue to function. Other forms of treatment include moving the diseased organism to another environment or removing the diseased parts from the organism. The most effective way to control disease is by preemptory prevention. The best method is to eliminate a disease-causing organism from the environment, such as by killing pathogens or parasites contaminating a water supply. Also effective is the disruption of a pathogen's transmission from one organism to another, either by avoiding contact with body tissues or fluids that harbour a pathogen or by eliminating an intermediary vector (e.g., killing the mosquitoes that transmit malaria to humans). Disease also may be prevented by removing a susceptible organism from an unhealthful environment, strengthening the organism's defenses by making it healthier, or vaccination. Additional reading Lawrie Reznek, The Nature of Disease (1987), written for the general reader, discusses the nature of disease from several perspectives, including medical, legal, political, philosophical, and economic. David O. Slauson, Barry J. Cooper, and Maja M. Suter, Mechanisms of Disease: A Textbook of Comparative General Pathology, 2nd ed. (1990), written for the veterinary student but a great resource for pathologists and biomedical researchers, provides a fundamental overview of the mechanisms of diseases, often at the molecular level. Max Samter (ed.), Immunological Diseases, 4th ed., 2 vol. (1988), covers the collagen diseases. F.M. Burnet, The Natural History of Infectious Disease, 3rd ed. (1962), offers a unique view of infectious disease as an ecological and evolutionary phenomenon. Books for the general reader include June Goodfield, Quest for the Killers (1985), exploring efforts to conquer several epidemic diseases; Andrew Scott, Pirates of the Cell: The Story of Viruses from Molecule to Microbe, rev. ed. (1987); and Peter Radetsky, The Invisible Invaders: The Story of the Emerging Age of Viruses (1991). William Burrows Dante G. Scarpelli