The epidemic is spreading infectious diseases, far exceeding the normal level of morbidity in this area. For the emergence of the epidemic required number of preconditions: violation of sanitary rules, there are sources of the infection vectors for vector-borne infections and susceptible population, insufficient prophylactic work of health authorities and other
Regular (minimum) incidence for this area and in historical terms is called sporadic. This is most often no connection between an individual cases. Only in respect of certain infectious diseases, such as influenza, sporadic cases is expressed quite a large number of cases.
The increased incidence limited to the small territory [single town, part of the town (settlement), dormitories, barracks and so on] and flowing short time, called the epidemic outbreak.
The spread of infectious diseases in a large area, sometimes in several countries or on several continents with a massive defeat of the population, is called a pandemic.
In cases where the burden of infectious disease in a particular area is logged continuously for many years, it is customary to speak of endemic or endemicity of a particular disease. If the incidence in a certain area registered a long time due to poor sanitary conditions (typhoid fever, dysentery in poor production of water supply and treatment, typhus with rampant), speak about so-called statistical endemicity. If this incidence is associated with the presence of certain natural conditions (constant circulation of the causative agent of plague among marmots, ground squirrels or gerbils, the presence in a particular area of guardians of infections, like a tick in the encephalitis or tick return typhoid , and so on), then we say about the true endemicity of the disease.
Infectious diseases, unusual, unusual for the area (country) and imported from other, more remote places, called exotic (for the USSR - smallpox and other).
In the movement of infectious diseases are observed quantitative fluctuations on the time of year (season). They are due to the influence of natural conditions. So, seasonal increases in morbidity can be defined seasonal fluctuations activity vector control (malaria, tick-borne encephalitis, mosquito fever and others), some features of animal life - sources of infection, such as during winter hibernation squirrels, marmots dropped possible contact of a person and thus prevents the appearance of the plague; mass breeding of mice in autumn-winter period and the appearance among them tularemia epizootic cause mass incidence of tularemia among the people. The appearance of fruit and vegetables, their consumption without observance of rules of hygiene entails an increase in intestinal infections, etc.

The epidemic (gr. epidemia, from epi - among and demos - people) is a high degree of intensity of epidemic process.
The epidemic process is a continuous process of transmission from the sick or diseased animals (sources of infection) to healthy people. By L. C. Gromashevskaya, epidemic process is a continuous chain following each other infectious conditions. The intensity of epidemic process, i.e. the degree of dynamism of transmission depends on the period of incubation of the disease (the shorter the term, the faster the process of new infections), activity and nature factors of transmission of the infection, susceptibility of the population to infection. All these elements make up the biological nature of the epidemic process and determine its intensity.
To social and biological factors affecting the dynamics of transmission include the number of people at risk of infection. When a single disease in the family or apartment probability of new infections is more or less limited. If the disease emerged in dormitory, school, kindergarten, orphanage, etc., that this probability is much greater. Bacterial contamination of the water supply system threatens the health and life immeasurably more people, etc.
Numerous other reasons, representing a combination of biological and social factors that also affect the dynamics of transmission. They can be divided into two groups. The first group consists of the constraints that slow down the transmission of infection: the small population areas; lack of natural foci transmissible infections; the stability of the local population; municipal accomplishment of settlements; favorable housing conditions and free accommodation in homes; quite good sanitary conditions at work; high level of sanitary culture of the population, including childcare providers, enterprises of food industry, catering and food trade; the possibility of a planned mass of specific prophylaxis of infections; high level of clinical and laboratory services to the population; well-organized and carried out by the hygiene and anti-epidemic work among the population (control of sanitary the status, working conditions and life of the population, utilities, childcare facilities, catering and sale of food products; well-conducted survey epidemic foci, work in areas of infection, detection and neutralization of carriers of infection, measures against the introduction infections from abroad, etc. and so on
The second group consists of factors that contribute to the acceleration of the process of transmission: the high density of the population in the given territory; the presence of natural foci transmissible infections; the mobility of the local population (permanent or periodic replenishment of the population at the expense of visiting and other places of the country or from abroad); failure of communal sanitation and overcrowding in dwellings; violations of sanitary regime at work; insufficient level of sanitary culture of the population; poor organization grafting of the case or the inability to carry out mass prophylaxis (no scientifically proven means of specific prophylaxis in a number of infections, the absence or lack of drugs for mass immunization and other); the unsatisfactory organization of clinical, laboratory and sanitary-epidemiological services. Among the factors that and another group this list is not limited.
There are the following degrees of intensity of epidemic process: sporadic cases, focal, epidemic outbreak, seasonal epidemic, the epidemic local or more widespread, the pandemic. Different degrees of intensity of epidemic process have a specific biological and social basis.
Sporadic cases characterized by registration in the locality single infectious diseases, which undoubtedly has among themselves sometimes quite far epidemiological link, why are sources of the infection with them very often cannot be found. Sporadic cases may be evidence of attenuation of the epidemic process that may depend on many factors, among which - the depletion of susceptible populations, wide application of tools specific prevention and progressive reduction in the number of long infections, growth sanitary culture of the population and public health improvement of dwellings and settlements, active identification of the primary foci of infection and timely and reliable their disposal, etc. Under these favorable conditions, the attenuation of the epidemic process can become sustainable and progress to full disappearance of diseases. But it can also be temporal - up period another activation relevant factors of transmission to breaches of specific prevention or sanitary regime in a broad sense.
Focal characterizes the intensity of epidemic process in the outbreak of contagious disease. The degree focality is determined by the number of diseases in epidemic focus (see). Sometimes epidemic in the hearth occurs simultaneously or within a short period of time, several homogeneous diseases. In other cases in the outbreak occur sequentially, one after the other, at intervals equal to the incubation period of this disease. There may be other ways of occurrence of diseases in the centers. At the modern level of anti-epidemic work, the incidence in the centers often limited to a single case. While disease you can think of is common to all cases the source of infection and food or water routes of transmission. In the second case, {subsequent diseases) the source of infection is the first patient in the focus at the domestic factors of transmission. The consistent increase in the number of cases in the same hearth characterized epidemiologist with the adverse party.
Epidemic outbreak is characterized by simultaneous or sequential appearance of disease among the people, linked by the common power supply, water supply, paragraph food supply, while contact with sick when airborne transmission, etc. but has a limited, local value.

Some authors believe that the term "epidemic outbreak" Laden; that such a group of diseases are not that other, as the epidemic. However, between these concepts, there is considerable difference. For example, a group of diseases of local importance among people who used the water from contaminated wells or from corrupted and polluted water column, it is impossible to distinguish from a widespread epidemic due to a malfunction of head of water supply treatment facilities when the disease appear simultaneously in many parts of the city. The degree of intensity of epidemic process, and therefore the number of cases in both cases is not the same.
Seasonal epidemics are characterized by annual growth in certain months of the year, the level of diseases to the maximum, followed more or less slow their decline to the level observed in the off-season. Periodicity seasonal epidemics associated with biological, climatic, and social factors that determine the activation of factors of infection, possibly with increased susceptibility to infection of the population or certain age groups, the potential for more frequent violations of hygiene and sanitary regime, especially in poor health culture considerable part of the population.
Thus, during seasonal epidemics may experience intermittent increase in the intensity of epidemic process. The leading cause seasonal epidemics can be addressed through targeted impact on the factors of transmission of the infection, susceptibility of the population and factors of the social order.
The epidemic is a high incidence of infectious diseases among the population, associated with biological and social factors. Among the last - war, famine, natural disasters, leading to the deterioration of sanitary and economic conditions of work and life of the population, increasing migration of the population. In these conditions, infectious diseases can spread over a vast territory and to appear there, where before they were small or non-existent. Along with such can occur, and local epidemics associated with extraordinary reasons (accidents sewage and water networks, faecal contamination of drains of open water bodies that are sources of drinking water, the descent into public waterways contaminated wastewater from infectious hospitals, the introduction of especially dangerous infections, and so on). Local epidemics under appropriate conditions can spread far beyond the territory of the initial appearance.
A high degree of intensity of epidemic process in the period of the epidemic is determined by the multiplicity of sources of infection, high susceptibility to infection of the population, the lack of reliable means of specific prevention, increasing the probability of a meeting with the infection at movement on transport, in public areas and depends on the character of the leading factors of transmission.
The pandemic is characterized by a wide spread of infectious diseases, covering the entire territory of the country, neighboring countries, and sometimes many countries of the world. The most famous pandemic Spanish flu, which affected a large part of the world in 1918-1920, the pandemic return and typhus fever in our country in those years. Can be considered a pandemic and the global spread of polio after the second world war.
The pandemic spread characteristic of the infections, which are extremely high susceptibility of the population, mostly with a short incubation period and airborne transmission mechanism, especially infections, leaving no lasting post-infection immunity. A typical example of such infections is an influenza pandemic which are repeated periodically.