Most West Nile cases in Colorado happen between July and September, though mosquitoes can live until the first snow or hard freeze.

Mosquitoes carrying the virus were first identified in Colorado in 2003. That year resulted in the most human cases of West Nile in state history, with 2,948. Sixty-six people died. In 2023, the state saw a significant number of cases again, with 634 cases identified, resulting in 51 deaths.