The world was rocked in late March 2009 when a new strain of Influenza A virus was first detected in Mexico, followed by cases in the United States. It is unknown whether humans have natural immunity. This novel strain consisted of genetic elements from four different viruses: North American swine influenza, avian influenza, human influenza, and swine influenza typically found in Asia and Europe. Infected pigs act as a mixing pot in which re-assortment occurs between flu viruses of several species, in this case pig and human.
