2009 swine flu pandemic

The 2009 swine flu pandemic was an influenza pandemic that lasted about 19 months, from January 2009 to August 2010, and was the second of two pandemics involving H1N1 influenza virus (the first being the 1918–1920 Spanish flu pandemic). First described in April 2009, the virus appeared to be a new strain of H1N1 that resulted from a previous triple reassortment of bird, swine, and human flu viruses and that further combined with a Eurasian pig flu virus, leading to the term "swine flu".

Some studies estimated that the actual number of cases including asymptomatic and mild cases could be 700 million to 1.4 billion people—or 11 to 21 percent of the global population of 6.8 billion at the time. The lower value of 700 million is more than the 500 million people estimated to have been infected by the Spanish flu pandemic. However, the Spanish flu infected a much higher proportion of the world population at the time, with the Spanish flu infecting an estimated 500 million people, which was roughly equivalent to a third of the world population at the time of the pandemic.

The number of lab-confirmed deaths reported to the World Health Organization (WHO) is 18,449, though the 2009 H1N1 flu pandemic is estimated to have actually caused about 284,000 (range from 150,000 to 575,000) deaths. 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 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 seems to be getting better and then relapses with high fever, as this relapse may be bacterial pneumonia.