🔗 Wikipedia: The 2009 H1N1 flu outbreak

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The 2009 flu pandemic or swine flu was an influenza pandemic that lasted from January 2009 to August 2010, and the second of the two pandemics involving H1N1 influenza virus (the first being the 1918–1920 Spanish flu pandemic), albeit a new strain. First described in April 2009, the virus appeared to be a new strain of H1N1, which resulted from a previous triple reassortment of bird, swine, and human flu viruses further combined with a Eurasian pig flu virus, leading to the term "swine flu". According to WHO, the laboratory confirmed death toll is more than 18,036. Meanwhile, some studies estimated that 11 to 21 percent of the global population at the time – or around 700 million to 1.4 billion people (out of a total of 6.8 billion) – contracted the illness. This was more than the number of people infected by the Spanish flu pandemic, but only resulted in about 150,000 to 575,000 fatalities for the 2009 pandemic. A follow-up study done in September 2010 showed that the risk of serious illness resulting from the 2009 H1N1 flu was no higher than that of the yearly seasonal flu. For comparison, the WHO estimates that 250,000 to 500,000 people die of seasonal flu annually.

Unlike most strains of influenza, the Pandemic H1N1/09 virus does not disproportionately infect adults older than 60 years; this was an unusual and characteristic feature of the H1N1 pandemic. Even in the case of previously very healthy people, a small percentage develop pneumonia or acute respiratory distress syndrome (ARDS). This manifests itself as increased breathing difficulty and typically occurs three to six days after initial onset of flu symptoms. The pneumonia caused by flu can be either direct viral pneumonia or a secondary bacterial pneumonia. A November 2009 New England Journal of Medicine article recommended that flu patients whose chest X-ray indicates pneumonia receive both antivirals and antibiotics. In particular, it is a warning sign if a child (and presumably an adult) seems to be getting better and then relapses with high fever, as this relapse may be bacterial pneumonia.