It's new! It's different! Or is it? New Scientist has put together an outline tracing the origins of the H1N1 influenza virus. Surprise: The first date is 1889, the year that jockeying between H1 and H2 variants of flu set the stage for the 1918 influenza pandemic. The virus involved in that was an distant relative of today's H1N1.
The Timeline does a great job of explaining how viruses evolve, how the interaction of humans and viruses in the past influences their "relationship" today, and exactly why older people have an immunity to the 2009 H1N1 swine flu that younger people lack. Some other interesting bits:
Swine flu is first isolated from a pig in Iowa.
An H1N1 virus appears in north-east China and starts circulating in humans. It causes seasonal flu in every subsequent year. No one knows where it came from, though it looks like an H1N1 that circulated in the Soviet Union in 1950 and some suspect it escaped in a laboratory accident.
The virus causes a mild flu pandemic, which mainly affects people born after H1N1 flu disappeared in 1957. However, the real surprise is that it does not displace the previous, and more virulent, seasonal flu, H3N2. Instead, it continues circulating alongside it.
The antibodies people produce after being infected by this new seasonal H1N1 do not protect against 2009 H1N1. However, infections also trigger another reaction called cell-mediated immunity, in which certain white blood cells target and destroy infected cells. Tests of the 2009 H1N1 pandemic vaccine show that, unlike antibodies, cell-mediated immunity to seasonal H1N1 may help protect against the pandemic virus. This does not prevent disease altogether, but can reduce its severity
The predecessor of the 2009 H1N1 swine flu virus emerges in the US. It is a hybrid of human, bird and swine flu viruses, and by 1999 it is the dominant flu strain in US pigs.
Thanks to Steve Silberman for pointing me to this!