West Nile virus was first recognized in the United States in 1999 in the state of New York. It is commonly found in Africa, West and Central Asia, and the Middle East. The virus was detected in Illinois in 2001. By January 2003, Illinois had the highest number of recorded human West Nile virus cases (877) in the U.S. including 62 deaths. This was not, however, the highest rate of human infection per capita in the county.

West Nile virus has a life cycle that primarily circulates between birds and mosquitoes. Other vertebrate hosts such as humans and horses are also affected, though they are thought to be "incidental hosts" as they do not help complete the virus life cycle. Preliminary data indicate that over 150 species of birds, as well as, horses, small mammals, reptiles and amphibians have been affected by WNV. In Illinois researchers detected WNV in birds, horses, squirrels, dogs, and a young zoo-raised wolf, as well as, mosquitoes and humans. In its native range, there are occasional flare-ups of WNV related illnesses, but in most years there is not widespread human heath concern.

While there are many unknowns regarding this virus, overreacting can cause many problems as well. Since many agents that could be used in mosquito control are not specific, they can kill many non-target species including beneficial insects. Mosquito populations can be monitored for WNV, and when present, a relatively mosquito-specific larvicide (kills the aquatic mosquito larva) can be used for control.

Is it only humans who become ill when infected with WNV?
No. Certain animals also can become ill when infected with WNV. For example, horses exposed to WNV can develop encephalitis. Also, certain birds – particularly crows, blue jays and certain hunting birds, such as hawks and falcons – can become ill and die with WNV infection.

If I travel to an area where birds with WNV have been reported and I am bitten by a mosquito, am I likely to get sick?
No. Even in areas where mosquitoes do carry the virus, very few mosquitoes (usually less than one out of 500) are infected. The chance that one mosquito bite will be from an infected mosquito is very small. Even a bite from an infected mosquito might show no or very minor symptoms in those with strong immune systems.