The virus that causes Eastern equine encephalitis (EEE) is a single-stranded RNA virus in the family Togaviridae, genus Alphavirus. It is endemic in North America, but only causes a few cases each year in the United States. EEE is spread by Culiseta melanura mosquitoes that feed primarily on birds. Horses and people are considered "dead end" hosts because the amount of virus in their blood is too small to infect mosquitoes (shown in the graphic below, courtesy of the CDC).
The life cycle occurs in swampy areas near hardwood forests. People are not common inhabitants of this habitat, so case numbers are small. Symptomatic infection is mostly seen in people under the age of 15 years and over 50 years. Most cases are seen in the Atlantic, Great Lakes, and the Gulf Coast regions.