2009 swine flu outbreak

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The 2009 outbreak of influenza A (H1N1) virus is an epidemic of a new strain of influenza virus identified in April 2009, commonly referred to as "swine flu." The source of the outbreak in humans is still unknown. Cases were first discovered in the U.S. and officials soon suspected a link between those incidents and an earlier outbreak of late-season flu cases in Mexico. Within days hundreds of suspected cases, some of them fatal, were discovered in Mexico, with yet more cases found in the U.S. and several other countries in the Northern Hemisphere. Soon thereafter, the U.N.'s World Health Organization (WHO), along with the U.S. Centers for Disease Control and Prevention (CDC), expressed concern that the A(H1N1) could become a worldwide flu pandemic, and WHO then raised its pandemic disease alert level to "Phase 5" out of the six maximum, as a "signal that a pandemic is at the imminent level".
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Although virologists have noted that the outbreak has proven relatively mild and less fatal than historic pandemics,[47] other health officials, including CDC Director Richard Besser, worry about what might happen later in the year, stating that "we are not seeing any sign of this petering out. We are still on the upswing of the epidemic curve. The number of cases is expected to rise as the new flu spreads across the country." [48]

The new strain is an apparent reassortment of four strains of influenza A virus subtype H1N1.[49] Analysis by the CDC identified the four component strains as one endemic in humans, one endemic in birds, and two endemic in pigs (swine).[49] However, other scientists have stated that analysis of the 2009 swine flu (A/H1N1) viral genome suggests that all RNA segments are of swine origin,[50] and "this preliminary analysis suggests at least two swine ancestors to the current H1N1, one of them related to the triple reassortant viruses isolated in North America in 1998."[51] One swine influenza ancestor strain was widespread in the United States, the other in Eurasia.[49]

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Historical context
Annual influenza epidemics are estimated to affect 5–15% of the global population, resulting in severe illness in 3–5 million patients and causing 250,000–500,000 deaths worldwide. In industrialized countries severe illness and deaths occur mainly in the high-risk populations of infants, the elderly, and chronically ill patients.[52]

In addition to these annual epidemics, the influenza A virus has caused three major global pandemics during the 20th century: the Spanish flu in 1918, Asian flu in 1957 and Hong Kong flu in 1968–69. These pandemics were caused by an Influenza A virus that had undergone major genetic changes, due to which the population did not possess significant immunity to the virus.The influenza virus has also caused several pandemic threats over the past century, including the pseudo-pandemic of 1947, the 1976 swine flu outbreak and the 1977 Russian flu, all caused by the H1N1 subtype.[53] The world has been at an increased level of alert since the SARS epidemic in Southeast Asia (caused by the SARS coronavirus).[57] The level of preparedness was further increased and sustained with the advent of the H5N1 bird flu outbreaks because of H5N1's high fatality rate, although the strains currently prevalent have limited human-to-human transmission (anthroponotic) capability, or epidemicity