I've been meaning to ask for a while...
That link above is a very good resource, however it describes something I've seen mentioned before: When an animal such as a pig which can be infected with human and animal forms of influenza does so, they say that a new "transgenic" strain can form. A ridiculous newspaper article I read early his year or late last year described the avian flu outbreak just before the SARS outbreak. It said that it traveled easily from birds to humans but not from human to human, but then said that it could "breed" with another virus which does spread from human to human and create a pandemic.
Umm, how can a genderless organism "breed?" They DID use that word. I'm sure they meant "transgenic shift" but how the heck could that work? I thought there was some "revolutionary" discovery a year or two ago when it was learned that Staph bacteria could swap genes. Why is it generally accepted without explanation?