Influenza has hundreds of strains. The virus mutates frequently, but virologists have divided it into three main categories: A, B, and C. Type A influenza is responsible for the majority of seasonal flu cases in humans and animals. Type B is another kind of flu that causes seasonal illness. It found only in humans. It is rarer and typically less severe than influenza A. Type C only affects humans but is much less severe than type A or B. Statistics suggest the flu is responsible for 200,000 hospital visits each year in the United States.

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