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Encephalitis or "inflammation of the brain" is a general term applied to a disorder which may have many causes. Usually this disorder is caused by a virus but it may also be produced by other types of micro-organisms. Encephalitis may also occur after an initial infection with mumps, measles and other childhood diseases, or following a vaccination, or as a result of a non-infectious process such as poisoning. Since encephalitis affects the brain, which in turn controls all body functions, encephalitis from any cause can show symptoms of general illness throughout the body such as headache, nausea and vomiting, fever, sore throat, and chills. Severe cases begin suddenly and severely and may be followed by stiff neck, drowsiness, trembling, mental confusion, convulsions and coma. Most encephalitis cases recover completely, but others suffer serious after effects such as mental deficiency, spastic or other paralysis, or emotional instability. Some cases die, with babies and the elderly being the most seriously affected. Treatment is symptomatic in most cases as antibiotics cannot kill viruses which are the most common cause of encephalitis. Why is encephalitis of interest to you, who are interested in birds, or for that matter, to me, as a medical entomologist? We have already discussed the many causes of encephalitis and that viral encephalitis is the most common. Viral encephalitis is transmitted in a variety of fashions including those forms transmitted by mosquitoes, and ticks, or the "arthropod-borne encephalitides." Of interest to us today are the mosquito-borne encephalitides, five forms of which occur in the United States. Although mosquito-borne encephalitides, has been known as a disease of man since the 1930's it has only been recently that any effort has been made to separate these encep-phalitides from those following other infections, and hence develop an accurate picture of the importance of mosquito-borne encephalitis.