What is West Nile Virus (WNV)?

West Nile virus is a serious disease most commonly spread by infected mosquitoes. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread the virus to humans and other animals. Symptoms (see below) are usually mild, but in rare cases, severe illness including encephalitis (inflammation of the brain), meningitis (inflammation of the lining of the brain and spinal cord), or even death, can occur.

Where is west nile virus found?

WNV has been found in all lower 48 states. Surveillance for West Nile virus in Illinois includes laboratory tests on mosquitoes, dead crows, blue jays, robins and other perching birds, as well as sick horses and humans with WNV-like symptoms. Surveillance for WNV in mosquitoes involves placing traps that attract and collect the mosquitoes that carry WNV. The mosquitoes are then tested for the presence of WNV in a laboratory. Mosquito surveillance data in suburban Cook County comes from the Cook County Department of Public Health, Northwest Mosquito Abatement District, North Shore Mosquito Abatement District, Desplaines Valley Mosquito Abatement District, South Cook County Mosquito Abatement District, and the Illinois Department of Public Health. See where WNV is currently circulating in mosquitoes.


Anyone living in an area where West Nile virus is present in mosquitoes can get infected. Serious illness can occur in people of any age. However, those at greatest risk are people that: are over 50 years of age; have certain medical conditions, such as cancer, diabetes, hypertension, kidney disease; have received organ transplants; and/or spend a lot of time outdoors; especially between dusk and dawn.

What are the symptoms?

Doctor examining patient

Four out of five people infected with WNV will not show any symptoms. Some people may become ill 3-15 days after being bitten by an infected mosquito.

Common symptoms include:

fever, nausea, headache and muscle aches.

Severe symptoms may include:

High fever, stiff neck, confusion, and muscle weakness.

What to do if you are experiencing symptoms:

Symptoms may last from a few days to a few weeks. Contact your primary healthcare provider if you are experiencing WNV symptoms.