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 department classified the risk of human infection as high as of the week ending Aug. 22. The cases are the first confirmed in suburban Cook County this season.

Last year was a bad one. Suburban Cook County recorded 44 human West Nile cases and four deaths in 2025, according to the county health department's announcement. Statewide, Illinois confirmed 150 human cases in 2025, more than double the 69 reported in 2024. Ten people died of the virus across Illinois in 2025.

Evanston falls within the department's suburban Cook County jurisdiction. The announcement did not identify which municipalities recorded the four cases.

Dr. Kiran Joshi, chief operating officer of the Cook County Department of Public Health, said the virus is active enough to warrant immediate action.

"In light of high West Nile virus activity in Cook County, it is incredibly important for people to protect themselves from mosquito bites," Joshi said in the department's announcement.

The virus had already been detected in mosquitoes collected in Northbrook and in DuPage and McHenry counties this summer, Patch reported. The Illinois Department of Public Health (IDPH) confirmed the state's first human case Wednesday, Aug. 26, in a Piatt County resident. IDPH also identified three probable cases in northern Illinois.

People older than 60 and those with weakened immune systems face the highest risk of severe illness, according to IDPH. Most people who contract the virus show no symptoms, the county health department said. Others develop fever, headache, rash and body aches three to 15 days after infection. Severe cases can cause brain inflammation, permanent neurological damage or death.

The county's "Fight the Bite" campaign recommends four steps:

• Use EPA-registered insect repellent containing 20% to 50% DEET for adults and 20% to 30% DEET for children.
• Eliminate standing water in flowerpots, old tires, buckets, birdbaths and plastic wading pools.
• Repair damaged window and door screens.
• Take extra precautions at dawn and dusk, when mosquitoes are most active.

West Nile virus has been detected in 50 Illinois counties so far in 2026. Residents can find more prevention information at FightTheBiteCookCounty.com.