Epidemics

EPIDEMICS (from Greek epi—upon, in, and demos—people), a more or less significant spread of certain contagious diseases among a population. Sometimes the word epidemic is also used to designate other mass non-contagious diseases where the cause is the simultaneous action on a mass of the population of a homogeneous harmful agent, e.g., pellagra, scurvy, ergotism (see), etc., and mass food poisonings. More precisely, an epidemic is called a condition when the frequency of morbidity from one or another contagious disease exceeds the limits usual for the given disease under given conditions (locality, population). Thus, with respect to certain "rare" diseases (plague, anthrax, etc.), the word epidemic is often used to designate even relatively small outbreaks numbering only 5–10 cases, which with respect to other, more widespread diseases (e.g., influenza) is treated merely as single cases. As long as cases are few, isolated, or do not exceed the frequency of diseases usual for a given locality, they are spoken of as sporadic cases, and only when this frequency becomes more significant is it spoken of as an epidemic. Finally, when an epidemic assumes exceptionally large, completely unusual proportions for a given infection, the concept of a pandemic is used (see). Epidemics by their origin can be of local, or "endemic," and imported, foreign, or "exotic" origin. The epidemic or mass spread of contagious diseases has long been that property of theirs which attracted universal attention (see Infection, Infectious Diseases). Microbiology has now established with certainty that the direct cause of contagious disease in humans and animals is always the penetration into the human or animal organism and development within it of parasitic living organisms (microbes). In the subsequent 60–70 years, research by scientists discovered the "agents" of most of the most common contagious or infectious diseases. Among the most important provisions firmly established by modern science is also the recognition of the specificity of these agents in various contagious diseases, as well as the significance of the effect on the living organism of those products which are the result of the vital activity of the parasitic agent. As a result of all these scientific conclusions, it has been established that an epidemic is the result of mass infection of people by homogeneous agents of one and the same contagious disease. But in contrast to old views of an epidemic as the result of the simultaneous action on the entire mass of the population of some externally acting cause ("miasma"), the cause of the mass nature of diseases among the human population during an epidemic must be viewed in those factors that facilitate the transfer of the agent (contagious principle) from one individual to another. The concept of infection, of the transmission of the contagious principle, becomes the fundamental element, the understanding of which is a prerequisite for the correct understanding of epidemics. Nevertheless, the tenacity of old views on the causes of epidemics as the result of the simultaneous action of a single external factor on the entire mass of the population still makes itself felt to the present time in the pseudo-scientific theories of some scientists standing on a semi-mystical or metaphysically-mechanistic point of view in the field of theoretical explanation of the causes of epidemic development. Examples of such theories in the field of the doctrine of the causes of epidemics can be served by the now abandoned, with the exception of its isolated followers, and at one time quite popular so-called localistic theory of Pettenkofer, which attempted to put the development of epidemics into a causal connection with the level of ground water; then, theories connecting the development of epidemics with various processes of an astronomical or cosmic order, linking the periodicity of epidemics with the appearance of sunspots, or finally explaining the same periodicity of epidemics by incomprehensible immanent laws of the epidemics themselves or mathematical laws of numbers, and so forth. All these theories even now from time to time are revived in the works of individual scientists paying tribute to the decadent moods of contemporary science of capitalist society, which doubts the possibility of solving complex scientific problems without the participation of transcendental factors. From the modern scientific views set forth above on the essence of contagious diseases, it follows first of all that the carrier of the contagious principle itself, which is by its nature a parasitic formation, must always be an infected person or animal; and therefore the first condition for the origin and spread of epidemics is always the presence of infected people (animals) carrying within themselves the agent of the given contagious disease. In those rare cases where people in a given locality fall ill as a result of contact with inanimate objects, the latter always bear agents on themselves as a result of prior contact with living carriers of infection. This latter method of introducing contagious diseases, contrary to a very widespread opinion, is encountered extremely infrequently, since the agents of contagious diseases, being parasites of the human (or animal) organism, find favorable conditions for their existence usually only in the living organism of their biological host, outside the organism of which only the most persistent of them are capable of surviving for a more or less prolonged time. And indeed this method of introducing an epidemic without the presence of living carriers has practical significance only in relation to anthrax, which is frequently introduced with animal raw materials (hides, wool) originating from animals that were ill with or even died from anthrax. But this phenomenon is possible thanks to the exceptional persistence of anthrax agents, associated with their ability to form spores outside the living organism. If infected people or animals are constantly present in a given locality, which actually takes place in relation to the majority of well-known contagious diseases, then such cases are considered as endemic; conversely, in cases where an epidemic is the result of the introduction of infection by individuals who arrived in an infected state from another locality, they speak of an epidemic of exotic origin (see above). For the appearance of an epidemic, however, the mere appearance of an infected subject in a given locality is not sufficient; it is also necessary that the contagious principle be able to be transmitted from the infected individual to others. This transmission in different diseases takes place through various paths. Modern epidemiology distinguishes in this regard 4 main groups of infectious diseases, each of which is characterized by more or less definite specific methods of infection. The first group includes intestinal diseases, in which the disease agent is located in the intestinal tract (typhoid fever, dysentery, cholera, many forms of helminthic diseases, etc.). The contagious principle is excreted by the infected subject outward together with feces, and enters the organism of another with food, drinking water, etc., necessarily through the mouth. No other mechanism for the transmission of contagion from the intestine of one subject to the intestine of another under natural conditions exists, and therefore only this method can have practical significance in the spread of epidemics of this group. The second group of infectious diseases consists of those in which the agent is located in the respiratory tract of the infected subject. Belonging here are, for example, measles, pertussis, diphtheria, scarlet fever, meningitis, pneumonia, influenza, pulmonary tuberculosis, and a number of other diseases. The contagious principle here can cause infection only after it has been removed from the respiratory system of the infected subject and then enters the respiratory organs of another subject. The role of an intermediary or intermediate carrier of the contagious principle for this group of contagious diseases is played by air. The third group of contagious diseases consists of those diseases in which the agent lives in the blood of the patient. Infection here can set in only in the event that the agent parasitizing the patient's blood enters the bloodstream of a healthy person. Belonging to the group of blood infections are malaria, typhus and relapsing fever, yellow fever, African sleeping sickness, filariasis, etc. The carriers of the mentioned diseases are blood-sucking parasites such as mosquitoes, lice, certain species of biting flies, and the like. Finally, the fourth group of contagious diseases includes those in which the contagious disease process, and consequently the very agents of the disease, are located on the outer surface of the body's integuments of the patient, on his skin, hair, or on open and semi-open mucous membranes. Well-known diseases of this group are, for example, scabies, trachoma, ringworm, favus, venereal diseases, etc. In these diseases as well, in the majority of cases, the transfer of the agent from the infected integuments of a sick person to the integuments of another occurs through the agency of various intermediate objects: trachoma via a towel, scabies via underwear. The difference is that this is the only group of contagious diseases in which the transfer of contagion from a sick to a healthy person is sometimes encountered through direct contact of the healthy organism with the infected one, without any participation of any intermediate intermediary (infection with venereal diseases through sexual intercourse).

Thus, for the development of an epidemic to be possible, there must also be a second prerequisite in the form of the presence in a given setting of that "vector" which is capable of playing this role specifically in relation to the given disease. The third prerequisite consists in the necessity of a human population susceptible to this contagious disease, or in other words, a disease pathogen virulent for this population. In the absence of this prerequisite, i.e., in a state where the ratio of the virulence of the pathogen and the non-susceptibility (immunity) of the population is such that the pathogen of the contagious disease, even if it has the opportunity to penetrate the human body, does not cause the development of the disease in it, or in such a ratio where susceptibility to the disease is preserved only in a few individuals, insurmountable barriers are placed in the way of the development of an epidemic: in the first case, the spread of the disease is impossible altogether, in the second, only rare sporadic cases are possible. An example can be smallpox, in relation to which complete or almost complete immunity of the population is achieved with the help of preventive vaccinations. The indicated three prerequisites (sources of infection, the presence of a factor serving as its vector, population susceptibility) could fully determine the course of epidemics only among the prehistoric biological ancestors of modern man or determine the spontaneous development of epizootics among modern wild animal species. In human society, the course of epidemics has become subject to the influence of social causes, which have taken over the function of regulating even this, as many mistakenly think, purely biological process. The constantly changing conditions of existence of human society, its technology, the basic forms of its economy dominating in each epoch, the way of life arising on their basis and the entire sum of cultural achievements, which often differed greatly within the depths of various historical formations, classes, professional, everyday and other groups, created the soil for a very diverse epidemic picture of the same contagious diseases in different historical periods, and in the same epoch—among peoples standing at different levels of their economic development, or among various social groups within a single country, state or part of the world. The entire history of epidemics undoubtedly points to the different development of epidemics in different periods of human history. Thus, leprosy, which in antiquity was extremely widespread among the population of the ancient cities of the feudal and semi-feudal East, at the beginning of the Middle Ages was brought to Europe and here, under similar conditions in the 11th–14th centuries, acquired an equally wide distribution. Further technical progress rapidly eliminated this infection in most European countries, and it is preserved at the present time in the form of mass diseases only where social and everyday conditions support this state of it (some countries of Asia, Africa). Cholera, which exists endemically in India, where it still produces hundreds of thousands of cases annually, was first brought to Europe in the first quarter of the 19th century, which can only be explained by the growth in the intensity of trade relations between Europe and India, and the course of cholera epidemics is determined by the direction and nature of these relations. Before the opening of the Suez Canal, all cholera epidemics reached Europe exclusively along the Volga via Astrakhan, which serves as the entry gate for this infection; after the aforementioned date, they go to Europe via Suez, Alexandria, and other ports of the Mediterranean Sea, being brought there by steamships coming from India. The same dependence of the development of epidemics on social and historical conditions is revealed by the history of the plague, which at all times of its spread followed certain processes of a social order, in turn accompanied by a sharp change in the conditions of settlement, movement, and reproduction of rats and other rodents serving as a permanent reservoir of plague infection. The ubiquitous spread of tuberculosis, characteristic of cities of the last historical epoch, where in fact the infection of the population becomes universal, is a consequence of that form of close communication between people, which was the product of this epoch with its form of housing structure and a number of other factors, with the full weight of their negative influences lying on the shoulders of the economically dispossessed and enslaved classes, which gave the disease itself the name "social" even under the dominance of the capitalist system. At the same time, it is clear from old descriptions that the pathological processes themselves basically retained their properties to such an extent that changes in the picture of their epidemic spread can by no means be reduced to the influence of any changes in the nature of the disease itself. No less characteristic is the fact of the completely different movement of epidemics among various social groups of the population of the same country and the same historical epoch. The influence of profession, class affiliation, material security, and other social factors is so sharply manifested in all epidemics of contagious diseases that these facts have received universal recognition. The spread of anthrax, which among the urban population affects almost exclusively persons working in the processing of animal products; the far from uniform morbidity of almost all infectious diseases among representatives of various class groups, according to statistics of capitalist countries, in accordance with their housing conditions, material security, literacy, and other social factors; the various forms and degrees of the spread of epidemics among the urban and rural population—all these are only isolated examples of the dependence of epidemics on social conditions mentioned above. The widely distributed concepts of water, food, milk, housing, and other epidemics, indicating the prominent role of the corresponding factors as spreaders of the contagious principle among a significant mass of the population during a given epidemic, in themselves already force us to recognize that such epidemics could arise precisely in the conditions of a certain social and everyday structure, causing a certain form of mass use by the population of water from one common source, milk or food obtained or prepared under conditions allowing their mass infection, etc. Finally, an important factor directing the movement of epidemics is the sum of those measures of a social and hygienic order that are carried out in relation to the prevention or termination of a threatening epidemic. Thus, in the presence of a source of infection, as well as a factor contributing to the transfer of infection from its carrier to others, with the susceptibility of the population to a given disease, there are prerequisites for the emergence of an epidemic, while the final question of whether an epidemic will occur or not, and if it occurs, what its size, nature of the course will be, to what territory it will spread, what groups of the population it will cover, etc., this question will be decided by the entire sum of social conditions that determine the forms of communication between people, and consequently the possible ways of spreading the contagious principle. In the development of an epidemic, one can observe its rise, when there are conditions contributing to this, and, conversely, a decline, when the causes feeding it are exhausted or eliminated, and as a result, the epidemic may cease altogether, but it can also leave infected material in the population, causing the preservation of an endemic source, which upon the resumption of a favorable environment can serve as a source for a new epidemic rise. The smaller the stock of means for combating epidemics at our disposal, the more the spontaneous nature of its emergence and development stands out, e.g., influenza epidemics. This spontaneous nature is most pronounced in contagious diseases of the respiratory organs, since the transmission of these diseases occurs through the air, and this path of transmission is still the most difficult to influence. The consequence of this is the circumstance that in cases where no serious obstacle (e.g., the immune state of the population, removal of the source of infection, etc.) is encountered in the path of the spread of a particular respiratory infection, the size of the epidemic can be very large, at least theoretically it can be expected that not a single individual will escape infection, i.e., in other words, as a result of such a process, the entire population susceptible to this infection will fall ill. Such a state of universal infection of the population is designated by modern German authors with the term "Durchseuchung", which some render in Russian by the word "proepidemichivanie" [pro-epidemicization]. But since the contracting of the infection ultimately leads to the development of immunity among those who have been ill, the very development of the epidemic contains the inevitability of its liquidation, and the period of rapid development of such a type of epidemic is inevitably followed by a period of lull or even its complete liquidation in a given locality or group of population. At the same time, a new occurrence of an epidemic of the same disease can occur only under the mandatory condition that a sufficiently compact mass of susceptible persons appears anew among the population, which requires a period of time usually calculated in years, and then it is only necessary for the source of infection to appear, and the epidemic arises again. Such a process is called the periodicity of epidemics and is characteristic exclusively of the group of respiratory infections.

In view of the same ease of spread of these infections, it is understandable that in their case, more than in any other infectious disease, the state of immunity is the regulator of human morbidity, and therefore, with regard to respiratory tract infections, the introduction of the concept of the so-called contagious index is of interest. This term denotes the percentage of susceptible individuals among those who have not previously suffered from the given infection, that is, in other words, what number of persons out of 100 exposed to a known danger of infection will actually fall ill. It is known that this index is highest for measles and equals 95-98%, for scarlet fever it reaches 35-40%, for diphtheria 15-25%, and for infections affecting the central nervous system (cerebrospinal meningitis, poliomyelitis, and epidemic encephalitis), which in their mechanism of transmission also belong to the group of respiratory tract infections, the contagious index is only 0.5-3.5%. It is clear what significance these indicators have for understanding the scale of the possible development of the corresponding epidemics. In the indicated property of respiratory tract infections, their most elemental character is manifested in comparison with all other infectious diseases, in which infectivity, as a rule, regulates the course of human morbidity to a much greater extent than susceptibility to them. Despite the fact that for many of these diseases the contagious index is 100% (e.g., plague, typhus and relapsing fever, venereal diseases, rabies, trachoma, etc.), in none of them is there observed the course of epidemics that is characteristic, for example, of measles, influenza, or smallpox among unvaccinated populations. In the vast majority of cases, we possess quite effective means of combating epidemics, which boil down mainly to eliminating the factors that determine their spread. These include the elimination, disinfection, and prevention of the source of infection (isolation and specific treatment of patients, freeing them from carrying the contagious principle in their bodies, e.g., in malaria, scabies, helminthic diseases, etc., the destruction of animals serving as the source of certain infections, e.g., rodents in plague, wolves and stray dogs in rabies, glandered animals, etc., sanitary border protection, etc.), the prevention of the transmission of infection from the infected to the healthy (destruction of vectors—lice, mosquitoes, flies, etc., disinfection, sanitary measures, etc.) and increasing the population's resistance to the given infection (preventive vaccinations). Lit.—see lit. to the article Epidemiology. L. Gromashevsky.