Three genetic systems of blood groups of great gerbils were studied to reveal that different resistance patterns exist in rodents toward Yersinia pestis and Salmonella typhimurium. Extrapolation of the results obtained on relations of co-members of parasite systems contributes to understanding of some points of immunobiological mechanisms of dynamics of expizootic procedures. A hypothesis is proposed claiming that isoantigenic blood structure of the host is responsible for formation of population resistance to infection.