Rock County Public Health confirmed the county’s first human case of West Nile virus this year on Monday, Oct. 5, and cautioned that mosquitoes can continue spreading the virus until a hard frost. West Nile is most common in Wisconsin in August and September, but the health department said the risk persists whenever mosquitoes are active. It described a hard frost as temperatures below 28 degrees for at least four consecutive hours. Infected mosquitoes can pass the virus to people, birds and other animals. West Nile does not spread directly between people, between animals, or from animals to people, according to the department. Most people infected with West Nile have no symptoms. Mild illness can include fever, headache, muscle aches, rash and fatigue. Fewer than 1% of infected people develop serious symptoms, which can include high fever, muscle weakness, stiff neck, confusion, tremors, paralysis or coma. Human cases have also been reported this year in Dane and Sauk counties. Rock County Public Health recommends using insect repellent containing DEET, picaridin, oil of lemon eucalyptus or IR3535; wearing loose-fitting long sleeves and pants; and considering rescheduling outdoor activities at dawn or dusk, when mosquitoes that spread West Nile are most active. Clothing can also be treated with permethrin, but the chemical should not be applied directly to skin.