Swine influenza refers to influenza caused by any strain of the influenza virus endemic in pigs. Strains endemic in swine are called swine influenza virus. Swine flu is common in swine and rare in humans. People who work with swine, especially people with intense exposures, are at risk of catching swine influenza if the swine carry a strain able to infect humans. However, these strains rarely are able to pass from human to human. Rarely, SIV mutates into a form able to pass easily from human to human. The strain responsible for the 2009 swine flu outbreak is believed to have undergone such a mutation. This virus is named swine flu because one of its surface proteins is similar to viruses that usually infect pigs, but this strain is spreading in people and it is unknown if it infects pigs.
The 2009 swine flu outbreak or H1N1 outbreak or North American influenza 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 an influenza-like illness were first detected in three areas of Mexico, but the new strain was not clinically identified as such until a month later 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., and several other Northern Hemisphere countries were also identified. By April 28, the new strain was confirmed in Canada, Spain, the United Kingdom, New Zealand and Israel and suspected in many other nations, including South Korea and Austria, with over 2,500 candidate cases, prompting the World Health Organization (WHO) to raise their pandemic alert level to 4. A level 4 warning means that the WHO considers that there is “sustained human to human transmission”; whereas levels 5 and 6 represent “widespread human infection”.
The new strain is an apparent reassortment of several strains of influenza A virus subtype H1N1, which analysis at the United States Centers for Disease Control and Prevention (CDC) identified as a strain endemic in humans, a strain endemic in birds, and two strains endemic in American and Eurasian pigs (swine).
In late April both the United Nations WHO and the U.S. CDC expressed serious concern about the situation, as it had the potential to become a flu pandemic due to the novelty of the influenza strain, its transmission from human to human, and the unusually high mortality rate in Mexico. On April 25, 2009, the WHO formally determined the situation to be a “public health emergency of international concern”, with knowledge lacking in regard to “the clinical features, epidemiology, and virology of reported cases and the appropriate responses”. Government health agencies around the world also expressed concerns over the outbreak and are monitoring the situation closely.
On April 24, 2009, Mexico’s schools, universities, and all public events were closed or suspended while other schools in the U.S. closed due to confirmed cases in students. Schools in Mexico were then announced to be closed until May 6, 2009.

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