Pathogenicity - the specific characteristic of the organism, as expressed in its ability to cause disease in the host's host in natural and experimental conditions. Pathogenic microbes, apparently occurred during the evolution of the relationship of saprophytic bacteria and mikroorganizmov by selection and long-term adaptation (adaptation) of saprophytes to a parasitic way of life in a living organism. Many pathogenic microbes on morphological and cultural properties very close to saprophytic representatives of the same family.

Pathogenicity (from the Greek. pathos is a disease and gennao - spawn), boleznennostew of microorganisms - it is their ability to induce clinical manifestations of infectious diseases. Pathogenicity is a relative concept, associated for each of the germ of a certain type, age, gender and physiological state of an organism in the tissue which it is embedded. The ability of a microorganism to respond to the disease on the action of live agents called susceptibility. The severity of disease-causing effects of the infecting microorganism different strains or even individuals microbe is determined by their virulence (see). Thus, pathogenicity can be caused both by increasing the sensitivity of an organism and increase of virulence of the infectious agent. So, for example, the herpes virus can long time to circulate in the body without causing any disease symptoms until then, until reduced resilience (i.e. increase of susceptibility) the body does not create conditions for pathological tissue damage and disease.
The pathogenicity of the micro-organisms arose in the process of evolution in the result of long-term adaptation to a specific group of hosts, and to the biological changes that occur in the tissues under the action of microorganisms and products of their vital activity (inflammatory exudates, necrotic changes and others). For the manifestation of pathogenicity requires certain conditions for each type of bacteria: the way of penetration, the availability of damaged tissues with impaired circulation and reduced redox potential, which creates conditions for the development of the dispute, anaerobic bacteria, and other
In some cases P. associated with damage to certain fabrics, other infectious agents are distributed throughout the body. Election localization pathogenic effect of many microorganisms due to the presence of certain enzyme systems (coagulase, streptokinase, desoksiribonukleaza, metazina and others) or toxins, and biochemical properties of the tissues in which microbes selectively deposited. For pathogenicity of the organism great importance is the presence on the surface of substances with necrobiotic properties; they damage the surrounding tissue, causing the release of active endogenous factors and yet suppress phagocytosis (see) and protective reactions of the organism [full antigens, antigens a, B and L pathogenic Escherichia coli, Vi-antigen bruchkomitsky bacteria, antigen M (mucoid) Salmonella, substance M hemolytic streptococci and other]. An increase or decrease P. may depend on biochemical changes occurring in tissues that form (or individual), which in normal conditions this organism is pathogenic. So, different species of malaria parasites, pathogens for certain animal species, cease to cause disease data animals, if you keep them exclusively for dairy diet (no milk para-aminobenzoic acid that is required for the development of appropriate types of Plasmodium).