It has been around for thousands of years.
Went looking for something else, but learned something I wasn't looking for: The origin of the virus dates back to 1937, when the virus was isolated from a woman in the West Nile District of Uganda. Thus, the name...
The intriguing part of WNV, though, is that a defector from Hussein's regime told us that his scientists had created a more deadly strain of the virus -- and he was planning to spread it in the USA. Then, the first summer after 9/11, we saw a serious nationwide outbreak of what had been a rare, exotic disease -- and it could be tracked directly back to near-simultaneous outbreaks centered on JFK and BWI, expanding rapidly from there in concentric circles.
In 1999, 62 diagnoses and 7 deaths
In 2000, 21 diagnoses and 2 deaths
In 2001, 66 diagnoses and 9 deaths
In 2002, 4156 diagnoses and 284 deaths
In 2003, 9862 diagnoses and 264 deaths
In 2004, 2539 diagnoses and 100 deaths
In 2005, 3000 diagnoses and 119 deaths
In 2006, 4269 diagnoses and 177 deaths
It wasn't especially deadly, but it was nasty enough. And it hardly existed in the USA prior to 2002.
I have a friend in Maryland who got it. It did not kill her but she has to live with the symptoms. It does not just go away once you get it.