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. The statements above, if true, most strongly support which one of the following? ['Some people who become infected with West Nile virus never show symptoms of illness.', 'West Nile virus is most common in those parts of North America with the highest density of mosquitoes.', 'West Nile virus will never be a common disease among humans.', 'West Nile virus was not carried to North America via an infected person.']