West Nile virus has been described in Africa, Europe, the Middle East, west and central Asia, Oceania (subtype Kunjin), and most recently, North America.

Recent outbreaks of West Nile virus encephalitis in humans have occurred in Algeria (1994), Romania (1996 to 1997), the Czech Republic (1997), Congo (1998), Russia (1999), the United States (1999 to 2003), Canada (1999–2003), and Israel (2000).

Epizootics of disease in horses occurred in Morocco (1996), Italy (1998), the United States (1999 to 2001), and France (2000). In 2003, West Nile virus spread among horses in Mexico.

In the US in 2002, West Nile virus was documented in animals in 44 states and the District of Columbia with Illinois, Louisiana, Michigan, and Ohio reporting the most deaths. By 2003, 45 states and D.C. had reported human cases.