Two people in eastern Wisconsin and one person in northeastern Illinois have died of West Nile virus, according to health officials.

A third person in Wisconsin also has been hospitalized because of the mosquito-borne illness, the Wisconsin Department of Health Services said Thursday in a release.

That state’s cases involve residents of Outagamie, Fond du Lac and Brown counties.

In Lake County, Illinois, three people tested positive for the virus over the past seven days, the Lake County Health Department and Community Health Center said Thursday in a release.

One of the victims experienced symptom onset in mid-August and died shortly thereafter.

West Nile virus is commonly spread through the bite of an infected mosquito. While most people don’t experience symptoms, about 1 in 5 can develop a fever, headache, body aches, vomiting, diarrhea or rash, according to the U.S. Centers for Disease Control and Prevention. About 1 out of 150 infected people develop a serious, sometimes fatal, illness.

An average of 18 cases of West Nile virus are reported in Wisconsin each year, and the virus has been detected this year in mosquitoes, animals and healthy blood donors whose blood screened positive for West Nile virus, the state said.

West Nile virus was first reported in the U.S. in 1999 in New York. It gradually spread across the country. In 2003, there were nearly 10,000 cases.