West Nile Virus Home > Cause of West Nile Virus
In most cases, West Nile virus is caused by the bite of an infected mosquito. In rare cases, infection occurs through blood transfusions, organ transplants, or breastfeeding.
In most cases, the cause of a West Nile virus infection is being bitten by a mosquito infected with the West Nile virus. In rare cases, a West Nile virus infection can be caused from blood transfusions, organ transplants, breastfeeding, and even mother-to-baby transmission during pregnancy.
The West Nile virus is a flavivirus (transmitted by mosquitoes and ticks) commonly found in Africa, West Asia, and the Middle East. It is closely related to the St. Louis encephalitis virus, which is also found in the United States. The West Nile virus can infect humans, birds, mosquitoes, horses, and some other mammals.
The length of time that the West Nile virus has been in the United States is unknown, but scientists believe the virus has probably been in the eastern United States since the early summer of 1999, or possibly longer.
Mosquitoes become infected with West Nile virus when they feed on infected birds. After transmission occurs, the virus may circulate in the blood for a few days. Infected mosquitoes can then transmit West Nile virus to humans and animals while biting. During blood feeding, the West Nile virus may be injected into the animal or human. The West Nile virus can then multiply in a person's blood and travel into the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.
Not everyone who gets bitten by an infected mosquito actually develops West Nile virus symptoms. Even if someone does develop symptoms, they are usually mild. In fact, less than 1 percent of people who get bitten and become infected will get severely ill. The chances you will become severely ill from one mosquito bite are extremely small.
However, there are certain factors that increase a person's chance of developing West Nile virus symptoms. These factors are known as West Nile virus risk factors. They include:
- Living in areas where virus activity has been identified
- Being over 50 years of age.
It is unknown if immunocompromised persons are at increased risk for West Nile virus disease.
(Click West Nile Virus Prevention for strategies on how to minimize the chance of becoming infected with the West Nile virus.)