diseases of man, animals, and birds caused by the entry of pathogenic microorganisms into organisms that are susceptible to the given infection. The principal feature that infectious diseases have in common is the capacity of an infected organism (or carrier of the disease) to transmit the disease to healthy individuals.

Among the many diseases that have afflicted man since ancient times, those that were widely diffused and transmitted rapidly from sick to healthy persons attracted particular attention. (That is why they were called all-embracing diseases or epidemics.) Some of these epidemics were accompanied by high mortality and were therefore called pestilence, pestilential disease, or plague. The occurrence of these diseases was attributed over the centuries to the action of peculiar miasmas, morbific vaporous exhalations. It was not until the 16th century that the Italian scientist Girolamo Fracastoro substantiated the theory of contagion and contagious diseases. The miasmatic theory prevailed until the founder of Russian microbiology, D. S. Samoilovich, proved that plague spreads by contagion and by a live causative agent.

The true nature of infectious diseases was revealed only in the 19th century after advances in bacteriology, microbiology, and immunology. These advances were the work of the French scientist L. Pasteur, the German bacteriologist R. Koch, and a galaxy of Russian scientists, including E. Metchnikoff, N. F. Gamaleia, S. L. Tsenkovskii, A. M. Bezredka, G. N. Gabri-chevskii, I. S. Savchenko, V. A. Khavkin, D. K. Zabolotnyi, and L. A. Tarasevich. Their work laid the foundation for the subsequent development of the theory and practice of controlling infectious diseases.

The term “infectious diseases” first began to be used in the 19th century for venereal diseases. It later came to include other diseases whose common feature was contagiousness. Advances in microbiology, virology, and immunology laid the foundation for the development of methods to control this group of diseases. The principal difference between infectious and noninfectious diseases is that in the infectious disease the organism and pathogenic agent possess definite biological activity. A pathogenic microorganism is a powerful irritant whose action provokes a highly complex process. This process consists from the very outset of two leading elements—the causative agent and the physiological condition of the organism. The relationship between these elements may vary in each specific case with the period, phase, or stage of development of the infectious process. Not only sick persons but convalescents, who discharge the causative agent for several weeks (diphtheria, intestinal infections), and carriers of infection may be sources of infection.

Infectious diseases are characterized by the regular development of the pathological process in which the following stages are distinguished: time of infection, that is, penetration of the pathogen into the body; latent or incubation period, that is, the period from the time of infection until the first symptoms of the disease appear; period of the premonitory symptoms of the disease (prodromal period), which is accompanied by general malaise, feeling of exhaustion, headache, slight elevation of temperature, and other such symptoms; period of intensification of the disease followed by the appearance of symptoms specific to each disease; period of the highest intensity of the disease, when all the characteristic symptoms are more or less fully manifested; period of abatement of the disease, when after a slow (lysis) or rapid (crisis) drop in body temperature, the clinical symptoms gradually disappear; and recovery period (convalescence), whose duration varies with the general properties and condition of the organism, severity of the disease, and conditions to which the patient is exposed. Complications may occur in the course of many infectious diseases (influenza, typhus, smallpox, diphtheria, scarlet fever).

After recovery, the individual usually acquires immunity of varying intensity and duration of insusceptibility to the causative agent of the particular infectious disease.

Rational treatment of infectious diseases consists in action directed against the main element in the infectious process, that is, the causative agent, and efforts to neutralize the toxins it produces.

Such specific therapy has been worked out only for certain diseases (for example, malaria, typhoid fever, relapsing fever, diphtheria, and syphilis). It entails the use of chemotherapy, vaccinotherapy, serotherapy, phagotherapy, and other forms of treatments. Antibiotics are also widely used. The treatment is strictly individualized. Steps are taken to correct some functional disturbances created by the pathological process (symptomatic treatment). Cardiovascular drugs, diuretics, laxatives, antipyretics, and vitamins are used for this. Treatment must be combined with observance of an antiepidemic regime (concurrent and terminal disinfection, vaccination of nursing personnel) along with the patient’s personal hygiene, good care, and proper diet. Deaths from many dangerous diseases have declined sharply as a result of the introduction of new methods of treatment (for example, antibiotic therapy). Such has been the case with epidemic meningitis, which at one time had a 50–60 percent mortality rate; now, owing to modern methods of treatment, it ends unfavorably only when conditions are unusually adverse. Pneumonic plague, which once was fatal in 98 percent of the cases, can now be successfully treated.

Infectious diseases are of social importance because they can spread widely (to epidemic proportions) and because they produce economic losses owing to partial or total incapacitation of the patients for more or less long periods of time.

Social and everyday factors—the economic and educational levels of the population, living conditions, work, and nutrition—play a decisive role in the spread of infectious diseases. That is why they are so prevalent in colonial and dependent countries and why the number of afflicted individuals and deaths is higher among the poorest strata of the population in the capitalist countries. For example, in India, 12,662,100 persons died of plague between 1898 and 1963. Between 1950 and 1963, there were 58,000 reported cases of plague in Asia, 3,000 in Africa, and 2,900 in America. During this same period there were 895,100 reported cases of cholera, 1,108,900 cases of smallpox (1953–63), and 711,500 cases of leprosy (1956–60) throughout the world. Some 30,100 persons died of influenza in Western Europe between 1955 and 1959. The most dangerous diseases (plague, cholera, smallpox, leprosy) are found mainly in Asia, Africa, and South America. Medical geography studies the spread of infectious diseases in different geographic regions.

Socialism creates all the conditions necessary to carry out medical and socioeconomic measures to control infectious diseases. Plague, smallpox, and mass cases of typhus, among other diseases, have been eradicated in the USSR. The incidence of such diseases as typhoid, diphtheria, and scarlet fever is much lower than in pre-revolutionary times. The USSR has frequently helped other countries control epidemics (of smallpox, poliomyelitis). For example, poliomyelitis vaccine made in the USSR was sent to 43 countries in 1959–60.