Eastern Equine Encephalitis
Eastern equine encephalitis occurs in the US, east of the Rocky mountains. It mainly affects young children and people over 55 years of age. This form has a higher mortality rate than the western variety.

Western Equine Encephalitis
The western equine encephalitis virus occurs throughout the US in all age groups. For an unknown reason, a high proportion of infections occur in children less than 1 year old.

Venezuelan Equine Encephalitis
This form of encephalitis is found from Florida through South America. At least 8 types of this virus have been identified.