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 .
extractive_keyphrases: remains died suddenly longer collapse berlin necropsy examination irritation drowned swelling suspected infection achim suffering showed celebrity died enclosure visitors possibly hundreds veterinary years virus believe pool infection encephalitis bear front two unclear found