West Nile virus was first recorded in the New World during August 1999 in New York City. Aetiology of the disease in the Old World indicated birds as the likely introductory and amplifying hosts with ornithophilous mosquitoes, e.g. Culex pipiens, as the principal vectors. Speculation regarding likely agents for movement of the virus in its new environment focused on migratory birds, but evidence to date is equivocal. While spread of the disease has been fairly rapid, at a rate of roughly 70 km a month, it has not shown the kind of long-distance, leap frog movements one might expect if transient birds were the principal introductory hosts. Furthermore, movement of the disease has not been focused southward, but shows a radiating pattern with detection sites located in all directions from New York where terrestrial habitat was available. In addition, tests among potential New World, avian hosts have revealed prolonged viraemia (up to 5 days) only in the relatively non-migratory House Sparrow (Passer domesticus). Dispersal movements by this species could account for the observed pattern of West Nile virus spread in the Western Hemisphere to date. Regardless of whether avian migration, dispersal, or some other agent is responsible, West Nile virus should reach the New World tropics in another 1-2 years, at which time a vast number of new potential introductory and amplifying avian hosts would be exposed to the disease and mosquito vectors would be available throughout most of the year, likely causing serious, long-term threats to human health and vulnerable avian populations in the region.