The first case of West Nile virus in Colorado in 2026 has been reported in Jefferson County.

Jefferson County public health officials announced on Tuesday that a resident of the county had contracted the virus, which is spread through the bite of an infected mosquito.

Most people who contract WNV do not feel sick, health officials said, but mild symptoms include fever, headaches, body aches and fatigue.

In 2025, 17 people died across Colorado from WNV, with 286 people contracting the virus, according to the Colorado Department of Public Health and Environment. Those numbers were huge increases from 2024, which saw only 76 cases.

Last year, Denver health officials reported 26 cases of West Nile. Jefferson County reported 23 cases, Adams County reported 27 and Arapahoe reported 18. Larimer, Boulder and Weld counties also had a large number of cases, with 53 cases reported in Larimer, 38 in Boulder and 34 in Weld.

Officials warned that despite the Denver area’s typically-dry weather, mosquitoes can still lay eggs in any standing water, such as buckets, gutters or birdbaths.

To stop the spread of WNV, health officials suggested residents use EPA-registered insect repellent when outdoors and wear long sleeves and pants at times with high mosquito activity, such as at dawn or dusk. Prevention steps also include ensuring window and door screens are in good condition and making sure to eliminate standing water that may accumulate around homes.

People who develop symptoms after being bit by a mosquito should contact their healthcare provider. People 60 and older with compromised immune systems are most at-risk for severe illness with WNV, health officials said.

Denver Gazette reporter Nicole C. Brambila contributed to this story.