influenza B -'bē n influenza that is usu. milder than influenza A, that may occur in epidemics but not pandemics, and that is caused by an orthomyxovirus (species Influenza B virus of the genus Influenzavirus B) infecting only humans and esp. children and having relative genetic stability with a single major serotype
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influenza caused by Influenzavirus B and occurring in epidemics at four- to five-year intervals.Medical dictionary. 2011.