Infection transmitted by mosquitoes
Typically, West Nile virus spreads to humans and animals via infected mosquitoes. Mosquitoes become infected when they feed on infected birds. You can't get infected by touching or kissing a person with the virus.
Most West Nile virus infections occur during warm weather, when mosquito populations 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 ranges from three to 14 days.
West Nile virus is present in areas such as Africa, parts of Asia and the Middle East. It first appeared in the United States in the summer of 1999 and since then has been found in all 48 contiguous states.
Other possible routes of transmission
In a few cases, West Nile virus may have been spread through other routes, including organ transplantation and blood transfusion. However, blood donors are screened for the virus, substantially reducing the risk of infection from blood transfusions.
There have also been reports of possible transmission of the virus from mother to child during pregnancy or breast-feeding, but these have been rare and not conclusively confirmed.