West Nile virus (WNV) is a mosquito-borne disease that was first detected in the United States in 1999. WNV first appeared in California in 2003, and by 2004, WNV had spread to all 58 counties. Risk of infection is reduced through the coordinated efforts of local and state public health and vector control agencies. People can protect themselves from WNV by taking precautions to prevent mosquito bites.