Birds and mammals can be infected with West Nile virus only through mosquito bites  Mosquitoes, in turn, become infected with the virus when they bite certain infected birds or mammals  The virus was originally detected in northern Africa and spread to North America in the 1990s
Humans sometimes catch West Nile virus, but the virus never becomes abundant enough in human blood to infect a mosquito
West Nile virus was not carried to North America via an infected person.