Influenza viruses pose a serious threat to humans. They mutate continually and rapidly. They also spread very quickly. Health authorities around the globe are worried the 2009 H1N1 virus could be particularly dangerous. The virus contains genetic material from flu strains that affect pigs,birds and humans. It can be transmitted from animals to humans and between humans themselves. Pigs can get the flu,just like people do. There's nothing unusual about that. Viruses can multiply and mutate very quickly in swine,and that can become a problem for humans.
Birds can also carry dangerous influenza viruses. Three years ago,there was a world-wide scare about an outbreak of bird flu that had crossed the species barrier and infected humans. But the situation now is somewhat different.