- Swine flu, also called Influenza A (N1H1) has been around a long time.  The 1918 Influenza outbreak that killed an estimated 20-50 million people was a type of the Influenza A (N1H1).
· As with all Influenza A, the current Swine flu virus originated in birds.  At some point it was transferred from birds to pigs and remained there for a period of time.
· In 1998, researchers discovered that the Swine flu H1N1 had combined with a human Influenza A virus and an American avian flu virus.
· The virus then spread to pigs in Asia where mutations occurred and variants developed that have been isolated around the world.
· It is not known where the virus was laying dormant and undetectable prior to the current outbreak.  It is possible that pigs in Mexico may have been this hidden reservoir where the virus built up a critical mass to make the transfer to the human host.

The CDC estimates that about 100,000 people probably have been infected with the Swine Flu H1N1 virus.

Reference
Science 22 May 2009:Vol. 324. no. 5930, pp. 996 - 997
cdc.gov