A simple virus that started in the belly of a dead bird is set to embark on a global killing spree. H5N1 - a bird flu virus with the potential to become humanised and mutate into the next pandemic flu virus. Horizon explores what could happen if a flu pandemic hits. The last flu pandemic in 1918 killed an estimated 50 million people worldwide. A virus today can spread much easier, much faster and there are estimates that hundreds of millions could be infected and potentially die.
