However, if an individual by virtue of the special conditions of life had no contact with the infection in childhood, it remains susceptible to it to old age (measles, chickenpox, smallpox). In other cases, (Diphtheria, scarlet fever). Thus, there is no real "childhood" diseases, and in fact is a consequence of age-sensitivity of domestic, rather than constitutional factors. This is clearly seen in the example of infectious diseases that affect, by contrast, mostly older. Most cases of typhoid fever a young age (20-25 years) only indicates a greater mobility (professional and consumer) representatives of this age, causing a higher chance of infection.

The sex composition of infectious patients does not show any clear trend constantly, and preferential loss of one or another floor due to consumer and professional factors. Most of the incidence of undulant fever in women on. Malta is understandable, because women more often and closer than the man who comes into contact with the source of infection – patients goats. Conditions or factors that make up the external environment in which man lives and under the influence of which it is located, are numerous and act collectively. We can briefly consider the effect of some of them. Meteorological factors, most often in the form of cooling, of course, plays a role in violating the body's resistance to infections. In relation to the anthrax and tetanus since the time of Pasteur's experiments proved that cooling plays a mordant for the manifestation of infection.

Nutrition plays an important role in maintaining the body's resistance to infections, and it may, the quantitative (starvation) or qualitative (beriberi), fatal affects the body's resistance. It is known that during starvation in general reduces the number of immune bodies, and some infectious diseases affect mainly people with low-powered (typhus and others). Some living conditions, in particular, crude, poorly ventilated dwellings with insufficient volume, reduce the resistance. Animals infected with TB and placed in an atmosphere with high carbon dioxide content, are killed before control. Sometimes the conditions of work (profession) may increase susceptibility to certain infectious diseases. It is shown, for example, increased incidence of tuberculosis spinners and stonemasons in capitalist countries, where occupational safety is not a constant concern of government and trade union bodies. Consequently, the socio-economic factors have a decisive influence on the outcome of the struggle of the human the body against infection.

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