Most often, WNV is spread by the bite of an infected mosquito. Viruses that are spread by mosquitoes are called “arboviruses.” Mosquitoes are WNV carriers (“vectors”) that become infected when they feed on infected birds. Infected mosquitoes can then spread WNV to humans and other animals when they bite.

All donated blood is checked for WNV before being used. The risk of getting WNV through blood transfusions and organ transplants is very small, and should not prevent people who need surgery from having it. Transmission during pregnancy from mother to baby or transmission to an infant via breast-feeding is extremely rare.

West Nile virus is not spread through casual contact such as touching or kissing a person with the virus, or by breathing in the virus.