Four adults in suburban Cook County have tested positive for West Nile virus, the Cook County Department of Public Health announced Friday, Aug. 28. The cases are the season's first in suburban Cook County and push the county's risk classification to "high."
The escalation hits close to home for North Shore residents. West Nile-positive mosquitoes have been found in every community served by the North Shore Mosquito Abatement District (NSMAD) since June, including Winnetka, Wilmette, Glencoe, Kenilworth and Northfield.
"In light of high West Nile virus activity in Cook County, it is incredibly important for people to protect themselves from mosquito bites … Using insect repellent, wearing long sleeves and pants, and eliminating standing water around your home can help protect you from mosquito bites," Dr. Kiran Joshi, chief operating officer of the Cook County Department of Public Health, said in the announcement.
The county classified the risk as high based on data through the week ending Aug. 22.
A summer of rising detections
The NSMAD, headquartered in Northfield, recorded its first positive mosquito batch of 2026 in Northbrook on June 9. Wilmette followed on June 16. By July 30, positive batches had turned up in Winnetka, Glencoe, Kenilworth, Northfield, Evanston, Glenview, Golf, Lincolnwood, Niles, Skokie, Deerfield and Highland Park.
NSMAD Executive Director Mark Clifton told the Chicago Tribune on June 19 that detections would climb as summer progressed. In 2025, the district recorded its highest infection rate on record, with about 56 out of every 1,000 mosquitoes testing positive at peak.
Statewide picture
The Illinois Department of Public Health (IDPH) confirmed the state's first human case of 2026 this week in Piatt County. Three additional probable human cases have been identified in northern Illinois, though specific locations have not been disclosed. West Nile virus has been detected in 50 Illinois counties so far this year.
Last year was severe. Suburban Cook County alone reported 44 human cases and four deaths in 2025. Statewide, Illinois logged 150 confirmed cases, more than double the 69 reported in 2024. Ten deaths were reported statewide in 2025.
Who is most at risk
Adults 50 and older face the highest risk of serious illness, according to the NSMAD. Most infected people develop no symptoms. Others experience fever, headache, rash and body aches three to 15 days after a bite. Severe cases can cause inflammation of the brain or spinal cord, permanent neurological damage or death.
How to protect yourself
IDPH's "Fight the Bite" guidance centers on three steps:
- Reduce standing water where mosquitoes breed. Empty flowerpots, birdbaths, gutters and any container holding stagnant water.
- Repel by applying EPA-registered insect repellent before going outdoors.
- Report areas where water has remained stagnant for more than a week to local government or the NSMAD.
The NSMAD continues weekly mosquito surveillance across the North Shore through the fall season.
