Influenza A viruses circulate in humans, birds, and pigs. Occasionally, different influenza A viruses can infect a single host at the same time. This often happens in places where humans and animals live in close contact. The genetic material of the viruses gets mixed up, creating an entirely new virus. (This process is called antigenic shift or viral reassortment.) If the new virus makes people sick and is transmitted easily from person to person, an influenza pandemic can occur.
That’s what happened in the spring of 2009: the virus that’s making the news right now has genes from flu viruses that normally circulate in pigs in Europe and Asia as well as avian (bird) and human genes.