CHAMPAIGN — West Nile virus activity has picked up in Champaign County, public health officials warn.

Mosquito samples tested positive for the virus recently in Champaign, Urbana, Savoy and St. Joseph, according to the Champaign-Urbana Public Health District.

Mosquitoes can be expected to remain active until the first hard freeze, said Jim Roberts, environmental health director for the district.

As of Sept. 22, there were 18 counties in Illinois with West Nile virus cases in mosquitoes, birds or people, plus one West Nile death in Cook County.

The virus spreads through the bites of infected mosquitoes, and health officials advise continuing to take precautions during the fall season while mosquitoes remain active.

Avoid being outdoors at dawn, dusk and in the early evening when mosquitoes are active and if you do go outdoors wear insect repellent. Make sure door and windows have tight-fitting screens and eliminate sources of standing water around your home.

Symptoms of West Nile virus sickness are usually mild and include fever, headache and body ache, but the virus can lead to more serious illness and death, with people over age 50 at highest risk for complications.