Officials say it's only a matter of time before they are found in the city.
West Nile virus is transmitted to humans by mosquitoes.
Crews are trying to keep the insect's population low by dropping larvacide into more than 200,000 catch basins. They're also putting mosquito traps throughout the city.
"These traps are checked regularly for the presence of infected mosquitoes. They help guide our efforts to spray insecticides, should the spraying become necessary," said Terry Mason, Chicago public health commissioner.
Last year, there were more than 20 human cases of West Nile virus in Illinois.
