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" by the media. 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".
Although virologists have noted that the outbreak has proven relatively mild and less fatal than historic pandemics,[56] 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." [57] In addition, the new virus strain could mutate over the coming months, leading to a new and potentially more dangerous flu outbreak later in the year.
The new strain is an apparent reassortment of four strains of influenza A virus subtype H1N1.[58] Analysis by the CDC identified the four component strains as one endemic in humans, one endemic in birds, and two endemic in pigs (swine).[58] 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,[59] 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."[60] One swine influenza ancestor strain was widespread in the United States, the other in Eurasia.[58]Our aim is to prevent swine flu,so wash your hands and keep clean.If you are not well,stay at home to rest.