Health officials say West Nile virus season is approaching as the weather warms up and mosquitoes become more active.
Kane County Health Department spokesman Paul Kuehnert says this year may be worse than the past two years when cool spring temperatures have helped slow the spread.
In Illinois last year, there were five human cases and no deaths.
This year, two birds have tested positive for the virus.
Health officials recommend avoiding being outdoors when mosquitoes are most active.
Officials say if venturing outdoors, wear long pants and long-sleeved shirts.
The virus is transmitted through the bite of a mosquito that has fed on an infected bird.
About 20 percent of people bitten by an infected mosquito will get sick.
The illness is usually mild but can be serious or fatal.