Oh wait, it never really left. It was a novel pathogen to the United States until it was introduced to New York in 1999. Then it spread across the entire country in 4 years and is now an established pathogen. The main reservoirs are birds although it has even been found in alligators. Culex Mosquitoes are the primary vector. If you notice lots of dead crows in your city, make sure you are using insect repellent. When it first spread through New York, birds, especially corvids, were dying in droves.
Remember to spray on the DEET and deter those little winged syringes from making a blood meal out of you! You may not be a demographic of high risk (young, old, or immunocompromised) but who wants to get a fever and nasty headache or take the risk of coma, paralysis, and death? For more information about the WNV, follow this link: http://www.cdc.gov/ncidod/dvbid/westnile/wnv_factsheet.htm