Viliuisk encephalomyelitis is an acute, often fatal, meningoencephalitis that 
tends to develop into a prolonged chronically progressive panencephalitis. 
Clinical, neuropathologic, and epidemiologic data argue for an infectious cause, 
although multiple attempts at pathogen isolation have been unsuccessful. To 
assess mechanisms of disease transmission and spread, we studied 6 multiplex 
families. Secondary cases occurred among genetically related and unrelated 
persons in a setting of prolonged intrahousehold contact with a patient 
manifesting the disease. Transmission to unrelated persons was documented in a 
densely populated region around the city of Yakutsk in which Viliuisk 
encephalomyelitis had not been previously known. Initially identified in a small 
Yakut-Evenk population on the Viliui River of eastern Siberia, the disease 
subsequently spread through human contacts to new geographic areas, thus 
characterizing Viliuisk encephalomyelitis as an emerging infectious disease.