Eastern equine encephalomyelitis (EEE), western equine encephalomyelitis (WEE), and Venezuelan equine encephalomyelitis (VEE) viruses are mosquito-borne pathogens that can cause nonspecific illnesses and encephalitis in equids (horses, mules, burros, donkeys and zebras) and humans in the Americas. Some of these viruses also affect birds and occasionally other mammals. No specific treatment is available, and depending on the virus, host and form of the disease, the case fatality rate may be as high as 90%. Epidemic VEE viruses are also potential bioterrorist weapons.