The present invention concerns the use as anti-viral agents of isolated fractions from berry juice of the plant genus Vaccinium. Two specific fractions are provided: (a) polymeric material having a molecular weight.gtoreq.12,000; and (b) proanthocyanidins-containing fraction.Thus, the invention provides methods of treating a subject against a viral infection; the method comprises providing said subject with at least one of the above isolated fractions in an amount effective to prevent production of a viral infection in said subject.A specific and preferred viral infection according to the invention is an influenza virus infection.