In adults stomach flu is caused most often by Norovirus, about 50% to 70% of cases. The new norovirus "Sydney 2012" is not associated with worse symptoms than other strains noroviruses. In the U.S., the Centre for Control and Prevention of Diseases show that Noroviruses cause 21 million illnesses annually. Of these, 70,000 cases require hospitalization and approximately 800 are fatal.
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