West Nile virus is most often spread by the bite of an infected mosquito and can infect many types of birds and mammals as well as people. Mosquitoes become infected when they feed on infected birds, which may circulate the virus in their blood for a few days. Infected mosquitoes can then transmit the West Nile virus to humans and animals while biting to take blood.
There has been documented transmission of West Nile virus through organ transplantation, blood transfusion, and transplacentally (mother-to-child). These modes of transmission represent a very small proportion of West Nile virus cases. The most common means of spreading the West Nile virus is through the bite of infected mosquitoes.