We have got used to hearing scare stories about strange strains of flu with names like H1N1, but is there any real danger? Jem Stansfield explains flu on a cellular level, showing how it can mutate from a cause of winter snuffles to a virus capable of killing millions. Liz Bonnin witnesses the vaccines being made that will protect us next winter. Maggie Philbin visits a lab where healthy patients are infected with flu, and witnesses exactly how it attacks.