West Nile Virus was first documented in Wisconsin toward the end of the summer of 2001. Presumably overwintering in mosquitoes or spreading from the east as was the case in 2001, it has reappeared in the Milwaukee area as well as northern Illinois.
Primarily a wild bird disease, West Nile Virus was confirmed in about a dozen humans in the U.S. in 2001, including an Atlanta woman who died. It has been found in about 80 bird species and 9 mammal species since its arrival in this country in 1999
The U.S. Geological Survey has been tracking the spread of the virus, in birds, from human infection reports, and in other modes.
You can monitor recent status via static maps on a web site (see URL below).
More information: West Nile Virus Maps from USGS
More information: Background on the West Nile Virus Project
Based on press releases from the U.S. Geological Survey