The story of West Nile virus in North America is a very recent one. The virus arrived in New York in 1999 and established populations in migratory birds on the Atlantic Flyway. it then systematically increased its population and spread to birds on the other primary migration pathways, moving steadily westward to the Mississippi, Central and by 2004, the Pacific Flyway. The virus infects birds of many species without causing visible illness, but cases fatal infections in Corvidae (crows, jays and their relatives). It can occasionally spill over and infect horses and humans, and can cause rare fatal results.