A scientist in the Division of Infectious Disease Laboratories at the Massachusetts Department of Health sorts mosquitoes according to species for testing in 2007. Only certain species can carry and transmit the Eastern Equine Encephalitis and West Nile virus. West Nile virus, found in tropical and temperate regions, infects mainly birds but is known to infect people, horses, dogs, cats, bats, chipmunks, skunks, squirrels and domestic rabbits.

The main route of human infection is through the bite of an infected mosquito. The virus first was isolated from a feverish adult woman in the West Nile district of Uganda in 1937 during research on yellow fever. Its first appearance in the Western Hemisphere was in 1999, with encephalitis reported in humans and horses. The subsequent spread in the United States might be an important milestone in the virus’s evolving history. (© AP Images)