document: Virus may have killed polar bear Berlin Zoo 's celebrity polar bear , Knut , drowned after swelling of his brain caused him to collapse and fall into a pool in his enclosure , a post-mortem examination has found . A necropsy of the four-year-old bear , who died suddenly two weeks ago , showed he was suffering from encephalitis , an irritation and swelling of the brain probably brought on by an infection . It remains unclear what that infection was , but Achim Gruber , a professor of veterinary medicine at Berlin 's Free University , said it could have been a virus . '' `` We believe that this suspected infection must already have been there for a long time ... at least several weeks , possibly months , '' '' Mr Gruber said . Knut died on 19 March in front of hundreds of visitors at Berlin zoo . Polar bears usually live 15 to 20 years in the wild and even longer in captivity .
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