First West Nile case of 2026 hits Colorado as officials warn even small pools of backyard water can breed infected mosquitoes

Colorado's first confirmed West Nile virus infection of 2026 has now been reported.

Public health authorities in Jefferson County said the infected person is a county resident.

Officials are warning residents to watch for water that collects around their homes, since even tiny amounts can give mosquitoes a place to breed.

What's happening?

A Jefferson County resident has tested positive for West Nile virus, making it Colorado's first reported case of the year, the Denver Gazette reported.

The new case comes after a sharp statewide increase last year. In 2025, Colorado recorded 286 West Nile infections and 17 deaths, compared with 76 cases in 2024.

Some of the largest county totals in 2025 were reported in Larimer, with 53 cases, Boulder with 38, Weld with 34, Adams with 27, Denver with 26, Jefferson with 23, and Arapahoe with 18.

Even though the Denver area is usually dry, mosquitoes can still lay eggs in small amounts of standing water left in places such as buckets, birdbaths, and clogged gutters.

Why does it matter?

Health officials say most people who get West Nile virus do not become sick. But if symptoms develop, they can include fever, headache, body aches, and fatigue, and some cases can become severe.

Adults 60 and older who have compromised immune systems are at the highest risk for serious illness.

A single reported case does not guarantee a major outbreak, but it does show that mosquito season is active.

What can I do?

Officials are encouraging residents to take simple steps to reduce their risk outdoors. That includes using an EPA-registered insect repellent, especially around dawn and dusk, when mosquitoes are often more active.

Wearing long sleeves and pants can also lower the chance of being bitten. At home, residents should inspect window and door screens and fix any damage that might let mosquitoes inside.

Officials also recommend getting rid of standing water wherever it gathers. Residents should empty buckets, clear gutters, and check for any other places where water can collect, because mosquitoes can breed in very small amounts.

Anyone who develops symptoms after a mosquito bite should contact a healthcare provider. That is particularly important for higher-risk people.

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