West Nile virus is carried by certain types of mosquitoes in Michigan. It is a potentially serious disease that can affect anyone, but people over age 60 are more likely to get the more severe form of WNV illness. The risk of bites from infected mosquitoes is highest for people who work or play outdoors. Wearing insect repellent when outdoors (especially at dawn and dusk) is important to prevent WNV.
Read more about it here: West Nile virus activity in MI