Interesting report out of Chile of a flock of turkeys infected with influenza A (H1N1) 2009. According to the ProMED report, more than 20,000 birds were infected.
Influenza viruses mutate regularly (this is described as antigenic shift or drift, depending on the extent of the mutation). This is why we get flu shots annually -- the shot you had last year may not cover the strain circulating this year. Viruses evolve. This is survival of the fittest at its most basic. One way that influenza viruses evolve is through the comingling of two strains within one host. So say a person is infected with a seasonal strain of influenza at the same time they are infected with the pandemic strain. The two viruses then have the opportunity to swap genes and create a new strain (please note that is a horribly simplistic explanation of the process).
One area of concern with viruses is their ability to jump species. This creates more opportunities for the virus to evolve.
Many species have their own influenza viruses. Dogs, for example, get a variety of flu strains which do not affect humans. Birds too. You may have heard about avian flu. A current strain is an H5N1 which has been circulating for a few years now. This H5N1 virus is highly pathogenic in birds -- this means it is highly lethal to them. It has also demonstrated the ability to infect humans, although on a relatively limited basis. It is highly pathogenic to humans as well. This virus has been the drive behind recent pandemic planning programs. We all assumed that the next pandemic would come from this strain, and the infectious disease community has been eyeing it closely. Human to human transmission of H5N1 has not happened on a widespread basis -- that virus does not appear to have the capability for rapid human to human spread.
H1N1 does though.
The rate of H1N1 spread has been unprecedented. It is now in most all corners of the globe. So what happens when a bird infected with H5N1 is coinfected with H1N1? Highly pathogenetic meets highly transmissible. I'm not liking the sound of that.