Viliuisk encephalomyelitis (VE), a progressive neurological disorder with a fatal outcome usually in several months to 6 yrs after disease onset, is seen only among the Iakut people of Siberia. The acute meningoencephalitic phase of the disease is followed by progressive dementia, rigidity and spastic tetraparesis. The disease is characterized by multiple micronecrotic foci with marked inflammatory reactions and gliosis in the grey matter. The acute febrile onset, with cerebrospinal fluid (CSF) pleocytosis and increased protein in the CSF, the epidemiology and the inflammatory neuropathology suggest the disease is infectious. Studies on household spread indicate an incubation time of up to several years. Viliuisk encephalomyelitis was restricted to an ethically distinct group of Iakut people of the Middle Viliui region, but in recent decades, with migration from this region, it has been spreading into previously unaffected Iakut populations. The occurrence of multiple VE cases in households and introduction of the disease by migrants into new populations indicate horizontal transmission in a setting of long intimate contact.