Mosquito-borne encephalitis/encephalomyelitis refer to viral diseases that are transmitted by mosquitoes. The term encephalitis refers to an inflammation of the brain; encephalomyelitis refers to an inflammation of the brain and spinal cord. In California, West Nile virus (WNV), St. Louis encephalitis (SLE), and western equine encephalomyelitis (WEE) are the three most important viral mosquito-borne viruses that can cause encephalitis/encephalomyelitis. The viruses that cause these diseases are maintained in nature through a mosquito-bird-mosquito cycle. Birds that are common throughout California, such as the American crow, English house sparrow and the house finch, are important carriers of these viruses. Horses are also particularly susceptible to infection with WNV and WEE, but there is a vaccine for horses to prevent these diseases.