Since its 1999 appearance in New York, West Nile virus (WNV) has spread relentlessly westward each year, opening up new fronts in the Midwest and the mountain states until pummeling California in the summer of 2004. The flavivirus, which is spread primarily by mosquitos, affects a variety of animals, including humans, horses, and nearly 300 bird species. As of October 15, 2004, about 940,000 Americans had been infected, of whom 190,000 became ill and 6790 developed WNV's most feared complications: neuroinvasive disease, including meningitis, encephalitis, and acute flaccid paralysis.1