Officials at the Chicago Department of Public Health are trying to find out how a 47-year-old man contracted West Nile encephalitis.
Department spokesman Tim Hadac says a check of all of the mosquito traps in the man's neighborhood have failed to turn up any evidence of the virus, The Chicago Daily Southtown reported Friday.
The victim, who became ill last month, is in a coma.
He is the sixth person to be diagnosed with West Nile in Illinois this year, the state's public health department said.
Last year, 10 people died of West Nile virus in Illinois and 215 human cases were reported.
West Nile virus is transmitted by mosquitoes that feed on infected birds.
Many people who are bitten by the virus-carrying mosquitoes fail to exhibit any symptoms while others develop fever, headache and body aches between three and 14 days after being bitten.
More serious cases progress to encephalitis and meningitis.
People older than 50 have the highest risk of becoming ill.
Copyright 2007 by United Press International
Explore further: Saudi Arabia: Deaths from MERS virus reach 348