Swine influenza refers to influenza caused by any virus of the family Orthomyxoviridae that is endemic to pig (swine) populations.
People who work with poultry and swine, especially people with intense exposures, are at risk of infection from these animals if the animals carry a strain that is also able to infect humans. SIV can mutate into a form that allows it to pass from human to human.
The 2009 swine flu outbreak is the spread of a new strain of H1N1 influenza virus that was first detected by public health agencies in March 2009.
Local outbreaks of influenza-like illness were first detected in three areas of Mexico, but the new strain was not clinically ascertained as such until a month later in cases in Texas and California, whereupon its presence was swiftly confirmed in various Mexican states and Mexico City; within days isolated cases elsewhere in Mexico, the U.S., the Northern Hemisphere were also identified.