Mosquitoes are responsible for many different diseases, mostly tropical, however some of which occur in Illinois. LaCrosse Encephalitis and St. Louis Encephalitis have historically caused human sickness in Illinois. Eastern and Western Equine Encephalitis are also present in the United States but are not yet important in Illinois. However, in 2010 a horse was euthanized from within the district with a probable case of Eastern Equine Encephalitis. Other diseases such as Malaria, Dengue Fever, and Yellow Fever are found in the US mainly by people traveling to the tropics. However, Illinois was once known as an area where Yellow Fever and Malaria were common. Draining swamps, mosquito control, and advances in modern medicine have delegated these diseases to mainly third world areas.
Bird Flu and HIV cannot be spread by mosquitoes to humans.
You are most likely to get West Nile late in the Summer. This is when the mosquitoes that spread the disease are most common. Even if mosquito populations are low and you are not being bitten at night you should still use insect repellent.