The Cook County Department of Public Health has reported the first four cases of West Nile virus in suburban Cook County this year and is urging residents to take precautions against mosquito bites.
All four cases were identified in adults, the health department said Friday.
Dr. Kiran Joshi, chief operating officer of Cook County Department of Public Health, urged residents to take steps to protect themselves from mosquitoes.
“Using insect repellent, wearing long sleeves and pants, and eliminating standing water around your home can help protect you from mosquito bites,” Joshi said in a news release.
West Nile virus occurs in humans when they are bitten by mosquitoes that contracted the virus from infected birds.
As of the week ending Aug. 22, the risk of human West Nile virus infection is considered high, the department said.
West Nile virus in Illinois
The Illinois Department of Public Health on Wednesday reported the state’s first human case of West Nile virus in Piatt County.
Mosquitoes carrying the virus were reported in Illinois in May, but no human cases had been reported at the time.
Suburban Cook County reported four deaths related to the virus in 2025, with 44 total cases reported last year, according to the department.
Instances of West Nile virus increased significantly in 2025 across Illinois compared to 2024, according to the department.
The state reported 150 confirmed human cases of the virus in 2025, more than doubling the 69 cases reported in 2024. Ten deaths were reported in 2025, compared with 13 deaths the year before.
Health officials warned that many people infected with West Nile virus have no symptoms and do not become ill.
Some may develop mild symptoms like fever, headache, rash, and body aches within three to 15 days after infection.
More severe symptoms can be neuroinvasive, impacting the nervous system, the brain or spinal cord. These symptoms can include high fever, stiff neck, confusion, muscle weakness, and inflammation of the brain, which can cause permanent neurological damage or death.
In June, CCDPH launched its annual “Fight the Bite” public awareness campaign to encourage residents to take precautions against West Nile virus.
How to prevent West Nile virus infection
The campaign encourages simple steps to reduce the risk of West Nile virus infection.
People are encouraged to use EPA-registered insect repellents to ward off the bugs.
Adults should use repellents containing 20% to 50% DEET, the department said. Child-friendly repellents containing 20% to 30% DEET are recommended for children.
Residents should also eliminate standing water from flowerpots, old tires, buckets, birdbaths and plastic wading pools where mosquitoes can breed.
Damaged windows and door screens can also let mosquitoes in. The health department urged people to check that their door and window screens fit tightly and do not have holes or gaps to protect indoor spaces.
Residents are also reminded to take caution and consider limiting outdoor activities at dawn and dusk when possible as mosquito activity is heightened during these hours.
To learn more about West Nile virus prevention, visit FightTheBiteCookCounty.com.