Definition of West Nile virus:

West Nile virus is a disease spread by mosquitos. The condition ranges from mild to severe.

Causes, incidence, and risk factors:

West Nile virus was first identified in 1937 in Uganda in eastern Africa. It was first discovered in the United States in the summer of 1999 in New York. Since then, the virus has spread throughout the United States.

The West Nile virus is a type of virus known as a flavivirus. Researchers believe West Nile virus is spread when a mosquito bites an infected bird and then bites a person.

Mosquitos carry the highest amounts of virus in the early fall, which is why the rate of the disease increases in late August to early September. The risk of disease decreases as the weather becomes colder and mosquitos die off.

Although many people are bitten by mosquitos that carry West Nile virus, most do not know they've been exposed. Few people develop severe disease or even notice any symptoms at all.

Risk factors for developing a more severe form of West Nile virus include:

• Conditions that weaken the immune system, such as HIV, organ transplants, and recent chemotherapy
• Older or very young age
• Pregnancy

West Nile virus may also be spread through blood transfusions and organ transplants. It is possible for an infected mother to spread the virus to her child through breast milk.