The isolation of viruses of tick-borne encephalitis, West Nile fever, and Omsk hemorrhagic fever from arthropods of nests of colonial birds (rook, sand martin, tree sparrow, Laridae) in different zones of West Siberia (from tundra to steppe) has been analyzed. The role of gamasid mites, hematophages and saprophages, characteristic inhabitants of nests of colonial birds, and of the tick Ixodes lividus in connection with their biology, coadaptation with hosts, microclimatic nest conditions, etc. has been evaluated. It has been concluded that the whole nest community of arthropods in a nest rather than one infected individual has to be regarded as an infective unit in colonies.