close up image of a mosquito biting

West Nile Virus

West Nile virus is spread to people by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes then spread West Nile virus to people and other animals by biting them. In Colorado, most human cases of West Nile virus are reported in August and September.

Symptoms of West Nile Virus

Most people infected with West Nile virus don’t have symptoms. However, one in five infected people will have flu-like symptoms, which usually begin two to 14 days after exposure. Serious, potentially deadly neurologic illness occurs in fewer than one in 100 infected people.

Who is at risk?

  • People aged 60 years and older and those with certain medical conditions—such as diabetes or kidney disease—are at greater risk of serious illness.

Protect yourself from West Nile Virus

  • Use an EPA-registered insect repellent, such as DEET, picaridin, or oil of lemon eucalyptus. The effectiveness of non-EPA registered insect repellents is unknown, including some natural repellents.
  • Limit time outdoors during the times of day when mosquitoes are most active, typically around dusk and dawn.
  • Wear loose-fitting, long-sleeved shirts and pants, especially when outdoors around dusk and dawn.
  • Use screens on windows and doors. Repair holes in screens to keep mosquitoes outdoors.
  • Frequently drain standing water from areas around the house such as bird baths, portable pools, tires, planters, pet water dishes, and children’s swimming pools. Mosquitoes can lay eggs and grow in standing water.
  • Treat standing water—such as ponds, ditches, clogged rain gutters, flowerpots, plant saucers, puddles, and buckets—with larvicide “doughnuts,” which can be purchased at hardware stores. Larvicide doughnuts use a naturally occurring type of bacteria to control mosquitoes.