Every community served by the North Shore Mosquito Abatement District has tested positive for West Nile virus this summer, and Illinois has recorded its first death from the disease in 2026.
The Northfield-based district detected virus-carrying mosquitoes in Wilmette, Winnetka, Glencoe, Kenilworth, Northfield, Evanston, Glenview, Northbrook and Skokie by Aug. 4. The Cook County Department of Public Health confirmed four suburban Cook County human cases on Aug. 28 and rated the risk of infection "high" as of the week ending Aug. 22.
The Chicago Department of Public Health announced the state's first West Nile death and the city's first two human cases on Tuesday, Sept. 1. The victim was a Chicago resident. The agency did not release the person's age or other identifying details. As of Friday, Sept. 4, at least 13 people had contracted West Nile statewide, the Chicago Tribune reported.
In 2025, suburban Cook County recorded 44 West Nile cases and four deaths. Statewide, 150 people were infected and 10 died.
Why the mosquitoes are worse in 2026
Chicago recorded 15.38 inches of rain from June 1 through Sept. 1, 3.32 inches above average, according to National Weather Service data. That made 2026 the 17th-wettest summer since 1871.
Chris Dietrich, a University of Illinois entomologist, told the Tribune that warm, wet conditions are "ideal" for flies and mosquitoes because faster life cycles in higher temperatures multiply their populations.
"Anywhere where even a little bit of standing water is accumulating, that's an ideal site for a female mosquito to go and lay some eggs in there and have the larvae develop," Dietrich said.
Kristina Lopez, a research entomologist at the North Shore Mosquito Abatement District, told WTTW on Aug. 31 that backyard containers are a major breeding ground. She pointed to tires, plant saucers, clogged gutters and children's toys that hold water.
What North Shore residents should know
West Nile virus spreads through the bite of an infected Culex mosquito, commonly called the house mosquito. It does not spread person to person. About one in five infected people develop fever and flu-like symptoms. Severe neuroinvasive illness strikes roughly one in 150 cases and is most likely in people over 60, those with chronic conditions and immunocompromised individuals.
The Asian Tiger mosquito, which has been biting more frequently this summer, does not carry West Nile, Block Club Chicago reported.
Illinois Department of Public Health Director Dr. Sameer Vohra said in a Sept. 1 statement that West Nile had been detected in 50 Illinois counties through positive tests in mosquito pools, horses or people. He urged older adults and immunocompromised residents to take precautions.
Health officials recommend wearing long sleeves and pants outdoors, using insect repellent and dumping any standing water around homes. Residents can check current mosquito surveillance data on the North Shore Mosquito Abatement District website at nsmad.org.
Mosquitoes and flies will die off after the season's first hard frost, which the National Weather Service says typically arrives in mid-to-late October.
