The H5N1 strain of avian flu isn't new. It was discovered in China in 1996. But in recent years it's started passing from mammal to mammal and it's now rife on cattle farms in the United States. How much should humans worry? David Aaronovitch speaks to: Professor Wendy Barclay, action medical research chair in virology at Imperial College London Kai Kupferschmidt, science journalist and molecular biologist Dr Caitlin Rivers, epidemiologist at the Johns Hopkins School of Public Health. Professor Ian Brown, avian virology group at The Pirbright Institute Production team: Sally Abrahams, Kirsteen Knight and Ben Carter Editors: Richard Vadon and Emma Rippon Production Co-ordinator: Gemma Ashman Sound engineers: Rod Farquhar