It is likely that swine influenza A (H1N1) virus has been evolving in Mexico for 10 years or so, picking up genes from “human”, “avian”, and “pig” viruses. It probably first infected humans along the USA/Mexican border in December 2008. It is likely that the Mexican epidemic began in January 2009 but remained undetected until April when scientists recognized that a new virus, different from the circulating H3N2 seasonal strain, was active in the community. At the same time, cases had also been detected in the US, and the potential for a pandemic was appreciated.