The pig is very topical, in a pandemic context, at the moment. The HP AI A/H5N1 (AKA Bird Flu) needs to accumulate a few genetic changes in its RNA sequence to adapt to man. We do not really know what most of these are but one that we do know is a modification to the H (in H5N1). An area on the Hemagglutinin called the RBS binds to a sugar on the host cell surface before it can enter the cell. There is a small configurational difference between the versions of the sugar the virus binds in birds and humans, this makes it difficult for avian flus to infect people. The bad news – for us and the pigs - is that pigs have both isomers and can catch both human and bird flus. They can then act as mixing vessel recombining and reassorting the genetic material to create new flu strains.