Eastern equine encephalitis (EEE) virus is a member of the genus Alphavirus, family Togaviridae. EEE virus has a single-stranded, positive-sense RNA genome. The virus particles are spherical and have a diameter of 60-65 nm. Of the four lineages of EEE antigenic complex, Group I consists of only EEE virus and is endemic in North America and the Caribbean and causes most human disease cases. The other three groups (IIA, IIB, and III) are caused by related Madariaga virus (formerly known as South American EEE virus) and cause primarily equine illness in Central and South America.