GRAND RAPIDS, Mich. (AP) -

Two people in western Michigan's Kent County have tested positive for the West Nile virus.

County health officials report Thursday that the cases involve unrelated adults. It's not known where they contracted the disease.

The health department says the virus is spread to humans primarily through the bites of an infected Culex species mosquito. Symptoms can include body aches, joint pain, and diarrhea.

Mosquitoes pick up the virus from infected birds and then pass it to people. The elderly and people with weak immune systems can be a greater risk for death.

Weekly  surveillance of the mosquitoes was conducted over three months in Kent County and testing was performed on up to 50 mosquitoes at a time. West Nile virus was found in 20 of those samples.