West Nile fever: here to stay and spreading.

The ultimate extent of West Nile virus's range in North America is uncertain but is likely to expand in 2001. Spread chiefly by night-biting Culex mosquitoes, the virus results in infection that most often is asymptomatic or causes a self-limited febrile illness. The elderly, however, are prone to develop neurologic manifestations, including potentially fatal encephalitis.