Human West Nile Virus Cases Grow in Illinois

Last week, the Lake County Health Department and Community Health Center in northern Illinois reported the county’s first human case of neuroinvasive West Nile virus this year. A Lake County resident in their 70s became ill in early July.

So far in 2025, nine out of 214 batches of mosquitoes have tested positive for West Nile virus in Lake County. West Nile virus typically peaks during the late summer in the area, usually from July through September when the mosquito population is at its peak. So far this year, the Illinois Department of Public Health has confirmed West Nile virus in batches of mosquitoes in 19 Illinois counties. 

In June, the U.S. Center for Disease Control and Prevention (CDC) confirmed Illinois’ first human case of West Nile virus of 2025 in an individual from southern Illinois who was hospitalized due to complications of the infection. About one in 150 people who contract the disease caused by a bite from an infected mosquito develop a severe illness that can affect the brain, spinal cord, and nerves and even cause paralysis.

The case marked the earliest onset of illness observed in Illinois since 2016, and underscored the risk posed by mosquito bites, especially to those who are over 65 or immunocompromised.

In 2024, Illinois confirmed 69 human cases of West Nile virus including 13 deaths, the highest number since 17 West Nile virus-related human deaths recorded in 2018.

As of July 15, the U.S. Centers for Disease Control and Prevention (CDC) has confirmed 49 cases of West Nile virus disease this year in 15 states. (These numbers do not include the cases in Illinois, and the CDC acknowledges cases are probably higher with delayed reporting.) West Nile virus neuroinvasive disease cases were confirmed in 24 cases. Last year, the CDC reported 1,791 cases and 164 deaths from the West Nile virus.