Don't believe me? It's antibiotics and staph all over again. Oh, and they can say MRSA is "community associated" all they want, but when I got a full body staph infection, due to which I had two lesions that had to be lanced and drained, I worked alone and sometimes went whole days without seeing another person. I certainly was not a football player. I mean, I know I'm not the healthiest person, but I was in my mid-20s and healthy enough to work 28 hours a week and be in college... Anyway.
Lest you become confused by the not-so-great explanation of antigenic shift in this video, here's a better description from this CDC article:
Influenza A viruses have eight separate gene segments. The segmented genome allows influenza A viruses from different species to mix and create a new influenza A virus if viruses from two different species infect the same person or animal. For example, if a pig were infected with a human influenza A virus and an avian influenza A virus at the same time, the new replicating viruses could mix existing genetic information (reassortment) and produce a new virus that had most of the genes from the human virus, but a hemagglutinin and/or neuraminidase from the avian virus. The resulting new virus might then be able to infect humans and spread from person to person, but it would have surface proteins (hemagglutinin and/or neuraminidase) not previously seen in influenza viruses that infect humans.Also, I can't find anything to really substantiate the idea that the viruses moved from birds to pigs to humans during all of the pandemics - that's really just a theory, and a recent one at that.
This type of major change in the influenza A viruses is known as antigenic shift. Antigenic shift results when a new influenza A subtype to which most people have little or no immune protection infects humans. If this new virus causes illness in people and can be transmitted easily from person to person, an influenza pandemic can occur.
View and enjoy!