This paper presents the authors’ opinion concerning such understudied issue of epizootology as the cause of the stability of the functioning of the natural focus of infection. The diversity of the biocenotic complex including main, secondary and casual carriers; numerous, small in number and rare vectors; strains with high virulence, low virulence and altered strains of the causative agent of infection, as well as population nonuniformity of biocenosis constituents contribute to the stability of the functioning of the natural focus in certain landscape-geographic conditions. The heterogeneity during the achievement of the abundance peak by various carrier and vector species also promotes to the stability. Universality of the principle of the proposition offered here for three- and two-component constituents of a natural focus of infection, as well as for infections caused by sapronotic pathogens, allows consider it as a biological law of the stable functioning of a natural focus of infection.