Public Health Director Dr. Eric Whitaker said the man was hospitalized with encephalitis July 11 and is now recuperating at his home in Jo Daviess County, in the northwest corner of the state.
The U.S. Centers for Disease Control and Prevention is performing tests to confirm the disease.
"It comes as no surprise that we have a human case of West Nile disease because of our state's past history with this disease, but the recurrence is a compelling reminder of the importance to all of us to take steps to prevent this disease," Whitaker said.
Illinois had 54 human cases of West Nile disease, including one death, in 2003. In 2002, the state led the nation with 884 human cases and 66 deaths.
West Nile is transmitted through the bite of infected Culex mosquitoes, but only about 20 percent of people bitten by an infected mosquito experience illness.