Causes

West Nile virus generally spreads to humans and animals from bites of infected mosquitoes. Mosquitoes get infected and carry the virus after biting infected birds. You can't get infected from casual contact with an infected person or animal.

Most West Nile virus infections happen during warm weather, when mosquitoes are active. The incubation period — the period between when you're bitten by an infected mosquito and the appearance of signs and symptoms of the illness — generally ranges from four to 10 days.

West Nile virus has appeared in Africa, Asia, Europe and the Middle East. It appeared in the United States in the summer of 1999, and since then has been reported in every state — except Hawaii and Alaska — as well as in Canada.

Other possible routes of transmission

In a few cases, West Nile virus might have spread through other routes, including organ transplants and blood transfusions. However, blood donors are screened for the virus, greatly reducing the risk of infection from blood transfusions.

There also have been reports of some transmission of the virus from mother to child during pregnancy or breastfeeding or exposure to the virus in a lab, but these are rare.