An undated photo provided by the Northwest Mosquito Abatement District in Wheeling, Ill., shows two varieites of mosquitoes. At the left is Culex pipiens, also known as the northern house mosquito. It is the specie mainly responsible for transmitting the West Nile virus to humans. At right is Aedes vexans, also called the floodwater mosquito and considered primarily just a nuisance mosquito. (AP Photo/Northwest Mosquito Abatement District)