Encephalitis caused by the eastern equine arbovirus, which is transmitted from horses to humans by mosquitoes; the incubation period is 1 to 2 weeks. Although this is the least common of the arboviruses, mortality is approx, 25%, and those who survive often have neurological problems. In the U.S. it occurs on the East Coast, Gulf Coast, and in the Great Lakes region during the mosquito season from midsummer to early fall.