We are faced with the disturbing possibility of a new influenza pandemic. However, it is still unclear whether the new form of H1N1 influenza, first identified in Mexico, will spread easily around the world, and if so, how many deaths will result.

As in the 1918 pandemic, which killed up to 50 million people world-wide, this new virus has inherited some of its gene segments from swine influenza; other genes have come from bird influenza and from an earlier human influenza, through a process known as gene re-assortment, or shuffling.