West Nile virus is a flavivirus, a family of viruses carried by mosquitoes and ticks. The virus is carried by mosquitoes and transmitted via a bite from an infected mosquito. It can infect humans, birds, mosquitoes, horses and some other mammals.

West Nile virus is commonly found in Africa, West Asia and the Middle East. It has also been found in Europe and more recently, in the United States and Canada.

It is not known how long it has been in the U.S., but scientists from the U.S. Centers for Disease Control and Prevention believe the virus has probably been in the eastern U.S. since the early summer of 1999, and possibly longer.

West Nile was first recognized in the United States in 1999 as the cause of severe illness and fatalities in humans in metropolitan New York City. The virus has since spread rapidly throughout the country.

• Infected mosquitoes. Most often, West Nile virus is spread by the bite of an infected mosquito. Mosquitoes become infected when they feed on infected birds. Infected mosquitoes can then spread the virus to humans and other animals when they bite.
• Transfusions, transplants and mother-to-child. In a very small number of cases, West Nile virus also has been spread through blood transfusions, organ transplants, breastfeeding and even during pregnancy from mother to baby.

An infected mosquito transmits the virus in its saliva via a mosquito bite. West Nile virus multiplies in the person's blood system and crosses the blood-brain barrier to reach the brain. The virus interferes with normal central nervous system functioning and causes inflammation of brain tissue.

The virus can cause several forms of disease in humans, with symptoms ranging from mild to severe.

• Serious symptoms in a few people. About one in 150 people infected with West Nile will develop severe neuroinvasive disease:
  • West Nile encephalitis: inflammation of the brain
  • West Nile meningitis: inflammation of the membrane around the brain
  • West Nile meningoencephalitis: inflammation of the brain and the membrane surrounding it.
  Severe symptoms can include high fever, headache, neck stiffness, stupor, disorientation, coma, tremors, convulsions, muscle weakness, vision loss, numbness and paralysis. These symptoms may last several weeks, and neurological effects may be permanent.

• Milder symptoms in some people. Up to 20 percent of the people who become infected have symptoms of West Nile Fever, such as fever, headache, body aches, nausea, vomiting and sometimes swollen lymph glands or a skin rash on the chest, stomach and back. Symptoms can last for as short as a few days, though even healthy people have become sick for several weeks.

• No symptoms in most people. Approximately 80 percent of people (about 4 out of 5) who are infected with West Nile virus will not show any symptoms at all.

People typically develop symptoms between 3 and 14 days after they are bitten by the infected mosquito.

There is no specific treatment for West Nile virus infection. In cases with milder symptoms, people experience symptoms such as fever and aches that pass on their own, although even healthy people have become sick for several weeks. In more severe cases, people usually need to go to the hospital where they can receive supportive treatment including intravenous fluids, help with breathing and nursing care.

There is no scientific evidence that people can be chronically infected with West Nile virus. What remains in a person's body for long periods of time are antibodies and "memory" white blood cells (T-lymphocytes) that the body produces to fight the virus. These antibodies and T-lymphocytes last for years, and may last for the rest of a person's life. Antibodies are what many diagnostic tests look for when clinical laboratories testing is performed. Both antibodies and "memory" T-lymphocytes provide future protection from the virus.

In the temperate zone of the world (i.e., between latitudes 23.5¹ and 66.5¹ north and south), West Nile cases occur primarily in the late summer or early fall. In the southern climates where temperatures are milder, West Nile virus can be transmitted year round.

No, but several groups are working towards developing a vaccine.

While its primary means of transmission is through infected mosquitoes, West Nile virus has been spread through blood transfusions in a small number of cases. In addition, blood donors could be infected with West Nile virus without knowing it, because:

• Approximately 80 percent of people (about 4 out of 5) who are infected with West Nile virus will not show any symptoms at all
• It can take between 3 and 14 days after infection for symptoms to appear