THE pandemic influenza (H1N1) virus (A/H1N1/pdm09- pdm09), which emerged in people in 2009, is thought to have been derived from pigs. The first cases of human infection with this virus were detected in North America, but the virus was retrospectively detected in people in Central America. Whether this is where the pandemic truly began or whether the virus was translocated to Mexico through infected people or animals still remains uncertain. The virus showed close similarity to progenitor strains circulating in pigs in both the Americas and Eurasia; it is genetically reassorted, with genes from these two distinct lineages. Although there are structured networks for surveillance of influenza in pigs, particularly in Europe and North America, coordinated surveillance is lacking at a global level. The results of surveillance programmes over time have detected changes in virus diversity through both genetic drift and genetic shift following the novel reassortment of viruses cocirculating within pigs, but also including viruses that are circulating in human and avian populations. Following the emergence of pdm09 in the human population there was a rapid dissemination of the virus across the world and, as a result of contact with infected humans, pigs became infected. The results of surveillance programmes in Europe and North America …