Omsk hemorrhagic fever (OHF) appears to be a unique nosological entity with respect to its clinical features, epidemiology, and ecology. OHF was initially diagnosed by the clinicians as an atypical form of typhus or generalized tularemia. Based on the pecularities of its clinical, epidemiologic, and pathoanatomic characteristics, however, it was classified as a unique disease by the end of 1946. Typical OHF has an abrupt onset, accompanied by fever, headache, myalgia, facial congestion, injection of the sclerae, and leukopenia. Discovery of significant immunity rates among water birds in the natural foci of OHF led to the study of mosquitoes. Collections were made in July and August of 1966 and 1968 in northern and southern forest-steppe regions. The role of birds in virus transmission has been intensively studied. In OHF foci, birds appeared to be free from tick infestation.