Key points
- West Nile virus is most commonly spread to people by the bite of an infected mosquito.
- Mosquitoes become infected when they feed on infected birds.
- People do not spread the virus, except rarely through blood transfusions, organ transplantation, and mother to baby.
Primary cause
West Nile virus disease is caused by a virus that is found in the United States and many other places in the world.
The virus belongs to a group of viruses called flaviviruses. St. Louis encephalitis virus and Powassan virus are two other flaviviruses that also circulate in the United States.
How it spreads
- West Nile virus circulates in the environment between mosquitoes (primarily Culex species) and birds.
- People become infected with the virus when mosquitoes feed on infected birds and then bite people.
- People are considered dead-end hosts because unlike birds, they do not develop high enough levels of virus in their bloodstream and cannot pass the virus on to other biting mosquitoes.
- The virus is rarely transmitted from person-to-person by blood transfusion, organ transplantation, and mother to baby, during pregnancy, delivery, or breastfeeding.
- Because the virus can be transmitted through blood and organ transplantation, people who were recently diagnosed with West Nile virus infection should not donate blood or bone marrow for 120 days following infection.
- Protect yourself and loved ones from West Nile by preventing mosquito bites.
For Public Health
Transmission of West Nile Virus