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In 1975 Ohio experienced the largest epidemic of mosquito-borne encephalitis in its history. Four hundred and sixty-eight human cases (416 St. Louis [SLE] and 52 California [CE] occurred with 30 fatalities; one death was due to CE. The entire United States was hard hit (Illinois, 475; Indiana, 297; Mississippi, 210; Michigan, 20; Tennessee, 91; 30 others, 550 ) with 1816 SLE, 160 CE, 133, Western, and 3 Eastern encephalitis cases causing 150 fatalities. The cycle of mosquito-borne encephalitis in nature progresses as follows: A mosquito infected with virus feeds on a non-infected animal, such as a bird in the case of St. Louis encephalitis. In approximately 24 hours the bird begins circulating the virus in its blood stream. This situation is called viremia. The bird circulates virus in sufficient quantity to infect other non-infected mosquitoes for a period of approximately four days. The virus then leaves the blood stream, and the bird is no longer able to infect a mosquito. After a period of approximately 10 days the virus has in turn invaded all tissues in the mosquito including the salivary glands. From that time on this mosquito can, by biting, infect any non-immune vertebrate.