The Centers for Disease Control and Prevention (CDC) and other health organizations are actively investigating the recent outbreak of 2009 H1N1 Flu Virus (Swine Flu) in humans. Cases in the U.S. were first reported in late March and early April 2009. CDC has determined that this swine influenza A (H1N1) virus is contagious and is spreading from human to human.

Swine Influenza is a respiratory disease of pigs (swine) caused by type A influenza virus that regularly causes outbreaks of flu in pigs. Like all influenza viruses, swine flu viruses change constantly. Pigs can be infected by avian influenza and human influenza viruses as well as swine influenza viruses. When influenza viruses from different species infect pigs, the viruses can reassort (i.e., swap genes) and new viruses that are a mix of swine, human and/or avian influenza viruses can emerge. There are four main influenza type A virus subtypes that have been isolated in pigs: H1N1, H1N2, H3N2, and H3N1, but most of the recently isolated influenza viruses from pigs have been H1N1 viruses. While swine flu viruses do not normally infect humans, sporadic human infections with swine flu have occurred. Most commonly, these cases occur in persons with direct exposure to pigs; human-to-human transmission of swine flu can also occur, as is the case with the 2009 outbreak.