In the northwestern region of Russia (Leningrad province) cattle is proved to be the main source of C. burnetii infection in humans, both in menaced professionals and in formally nonmenaced groups. Liquidation of specialized cattle-breeding complexes (with their well-organized veterinary surveillance) and broadening of the circle of non-professionals that contact with agriculture or domestic animals infected with C. burnetii provide the prerequisites to Q fever spreading among various groups of population.