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disease,impairment of the normal state or functioning of the body as a whole or of any of its parts. Some diseases are acute, producing severe symptoms that terminate after a short time, e.g., pneumonia; others are chronic disorders, e.g., arthritis, that last a long time; and still others return periodically and are termed recurrent, e.g., malaria. One of the most common bases for classifying disease is according to cause. External factors that produce disease are infectious agents, including both microscopic organisms (bacteriabacteria
[pl. of bacterium], microscopic unicellular prokaryotic organisms characterized by the lack of a membrane-bound nucleus and membrane-bound organelles. Once considered a part of the plant kingdom, bacteria were eventually placed in a separate kingdom, Monera.
..... Click the link for more information. , virusesvirus,
parasite with a noncellular structure composed mainly of nucleic acid within a protein coat. Most viruses are too small (100–2,000 Angstrom units) to be seen with the light microscope and thus must be studied by electron microscopes.
..... Click the link for more information. , and protozoansprotozoan
, informal term for the unicellular heterotrophs of the kingdom Protista. Protozoans comprise a large, diverse assortment of microscopic or near-microscopic organisms that live as single cells or in simple colonies and that show no differentiation into tissues.
..... Click the link for more information. ) and macroscopic ones (fungiFungi
, kingdom of heterotrophic single-celled, multinucleated, or multicellular organisms, including yeasts, molds, and mushrooms. The organisms live as parasites, symbionts, or saprobes (see saprophyte).
..... Click the link for more information. and various parasitic wormsworm,
common name for various unrelated invertebrate animals with soft, often long and slender bodies. Members of the phylum Platyhelminthes, or the flatworms, are the most primitive; they are generally small and flat-bodied and include the free-living planarians (of the class
..... Click the link for more information. ). Only infectious diseases can be transmitted—by humans, certain animals and insects, and infected objects and substances (see communicable diseasescommunicable diseases,
illnesses caused by microorganisms and transmitted from an infected person or animal to another person or animal. Some diseases are passed on by direct or indirect contact with infected persons or with their excretions.
..... Click the link for more information. ). Other external agents that can cause disease are chemical and physical agents (drugs, poisons, radiation), which can be encountered in specific work situations, deficiency of nutrients in the environment, and physical injury. Diseases that arise from internal (endogenous) causes include hereditary abnormalities (disorders inherited from one or both parents), congenital diseases (disturbances in the development of a normal embryo), allergies (hypersensitive reactions to substances in the environment), endocrine disorders (generally either overfunctioning or underfunctioning of an endocrine gland), circulatory disorders (diseases of the heart and blood vessels), and neoplasms, or tumors (masses of abnormally proliferating cells). Degenerative diseases occur as a result of the natural aging of the body tissues. Finally, a wide range of diseases are attributed to, or at least influenced by, emotional disturbances. Psychoses and neuroses result in disturbed behavior; the so-called psychosomatic diseases (certain kinds of colitis, many forms of headaches) are thought to be brought about by emotional stress. Most diseases occur as a result of a combination of both internal and external conditions, i.e., an interaction between the body and the environment. Thus a person may be hereditarily predisposed to tuberculosis, although the tubercule bacillus (the infectious agent) must be present for the disease to occur. In ancient times disease was ascribed to supernatural, spiritual, and humoral factors. The discovery by Louis Pasteur and others of the role played by microorganisms in infection and the study of cellular pathology by Rudolf Virchow in the 19th cent. were of the utmost importance in establishing the true nature of disease.
a process arising as a result of an (extremely) injurious external or internal stimulus acting on the body, and characterized by lowered resistance to the environment with simultaneous mobilization of the body’s defenses.
Disease is manifested by disruption of the equilibrium between the body and its surrounding environment. This disruption is revealed by the occurrence of side (inadequate) reactions and, in man, by a decrease in work capacity for the duration of the disease.
The general concepts of disease changed in the course of the history of medicine. Hippocrates thought disease was caused by an incorrect mixing of the four main fluids of the body: blood, mucus, yellow bile, and black bile (venous blood). About the same time the idea arose from the atomistic teaching of Democrites that disease results from a change in the shape of atoms and their incorrect arrangement. At the beginning of the Christian Era and especially in the Middle Ages, idealistic views were advanced to account for disease. According to these views, the soul or a special kind of vital force (arche) was responsible for the struggle of the body against the changes caused by disease. Avicenna (ibn Sina) proposed materialistic views of disease in the Middle Ages (for example, the origin of disease under the influence of invisible beings and the role of the constitution of the body).
From the 17th through the 19th century, important contributions to the theory of disease were made by G. B. Mor-gagni (the idea that disease is caused by anatomical changes), M. F. X. Bichat (description of the pathologicoanatomical picture of several diseases), R. Virchow (theory of cellular pathology), C. Bernard (disease as a disruption of the body’s physiological equilibrium with the environment), and others. S. P. Botkin, V. V. Pashutin, I. P. Pavlov, and A. A. Ostroumov related disease to impairment of the conditions of man’s existence and elaborated on the theory of nervism.
Despite the abundance of works on disease, the concept has still not been precisely determined. Some investigators deny that disease is qualitatively different from health. A. A. Bogomolets, for example, believed that disease does not create anything essentially new in the body. Others include only biological phenomena in the concept of disease. According to P. D. Gorizontov, disease is a complex general reaction to disruption of the relations between the organism and the environment. Disease results in the development of pathological processes reflecting local manifestations of the organism’s systemic reaction. I. V. Davydovskii maintains that there are no fundamental differences between physiology and pathology. Pathological processes and disease, in his opinion, are merely characteristics of the adaptive processes associated with subjective suffering. According to H. Selye’s concept of the general adaptation syndrome, disease is stress resulting from an extreme stimulus acting on the body.
The causes of disease are varied, but they can all be reduced to groups of mechanical, physical, chemical, biological, and, for man, also psychogenic factors. Inadequacy (extremeness or unusualness) in any of these factors induces disease. Inadequacy may be quantitative (the amount of the stimulus is excessive for the body), qualitative (the body is exposed to a factor for which it has not developed protective or adaptive mechanisms), temporary (a quantitatively and qualitatively adequate stimulus acting for a long time or at intervals or in rhythms unusual for the body), or dependent on the individual properties of the particular organism (that is, determined by the individual reactivity of the particular organism in the form of increased sensitivity). The social inequality and national oppression found under capitalism cause a number of diseases and even lead to the extinction of entire peoples—for example, native Australians, bush-men, and American Indians.
Modern thought characterizes disease by the following main features:
(1) The development of disease is greatly influenced by the external environment and, in the case of man, primarily by the social environment. Changes in the internal properties of an organism, caused by environmental factors and firmly fixed (including those fixed by hereditary mechanisms), may themselves eventually play a leading role in the development of disease.
(2) Besides the etiological factor (that is, the cause) and external conditions, the body’s protective and adaptive mechanisms play an important part in the development of disease, which is largely dependent on the efficiency of these mechanisms and the extent and rate at which they become involved in the pathological process. In man, the development and course of disease is greatly affected by the psychogenic factor.
(3) Disease is an ailment of the whole organism. There are no completely isolated diseases of organs and tissues—that is, local diseases. Any disease more or less affects the entire body, but this does not preclude the principal affection of a particular organ or part of the body.
The following periods of a disease are distinguished: the latency period (the incubation period in the case of an infection), the time—which may last from a few seconds (in poisonings caused by a powerful poison) to dozens of years (in leprosy)—between the initial action of the pathogenic agent and the appearance of the first symptoms; the prodromal period, marking the appearance of the first symptoms, which may be vague and nonspecific (elevated temperature, grogginess, and general malaise) or sometimes typical of a particular disease (Filatov-Koplik spots in measles); the period of full development, ranging from several days to dozens of years (tuberculosis, syphilis, and leprosy); and the period of culmination (recovery, convalescence), which may occur rapidly, as in a crisis, or by lysis.
Diseases may be acute or chronic, depending on the duration of their course and the rapidity of intensification and disappearance of the symptoms. The appearance of additional changes that are unrelated to the direct cause of a disease, alongside its principal manifestations, is called a complication. It may arise at the height of a disease or after the principal manifestations have disappeared. Complications are an aggravating factor, and they sometimes lead to an unfavorable outcome. A disease may end in complete recovery, recovery with residual phenomena, stable changes in organs, the occasional development of new manifestations of the disease in the form of long-range sequelae, and death. Death as the culmination of a disease may occur suddenly, after a brief death struggle, or gradually, after a more or less prolonged agonal state.
Human diseases are classified according to the nature of the course (acute or chronic), the level at which specific pathological changes occur (molecules, chromosomes, cells, tissues, organs, or entire organisms), the etiological factor (mechanical, physical, chemical, biological, or psychogenic), the method of treatment (medication, surgery, and others), age- or sex-related differences (gynecologic, pediatric), and so on. The nosological principle—that is, the classification of diseases based on a grouping by related symptoms—is the most widely used. It is necessary to note that none of the existing classifications is completely satisfactory. According to the nosological principle, pneumonia, for example, may be classified as a respiratory disease, an infection, and an allergy. Therefore, it is a major task of theoretical and practical medicine to establish a modern classification of diseases.
REFERENCESGorizontov, P. D. Voprosy patologicheskoi fiziologii ν trudakh I. P. Pavlova. Moscow, 1952.
Davydovskii, I. V. Problemy prichinnosli ν meditsine. Moscow, 1962.
Petrov, I. R., and V. B, Lemus. “Obshchee uchenie o bolezni.” In Mnogotomnoe rukovodstvo po patologicheskoi fiziologii, vol. 1. Moscow, 1966.
V. A. FROLOV