The
2009 flu pandemic or
swine flu was an
influenza pandemic, and the second of the two pandemics involving
H1N1 influenza virus (the first of them being the
1918 flu pandemic), albeit in a new version. First described in April 2009, the virus appeared to be a new strain of H1N1 which resulted when 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". Unlike most strains of influenza, H1N1 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 will develop
pneumonia or
acute respiratory distress syndrome (ARDS). This manifests itself as increased breathing difficulty and typically occurs 3–6 days after initial onset of flu symptoms. The pneumonia caused by flu can be either direct viral pneumonia or a secondary bacterial pneumonia. In fact, a November 2009
New England Journal of Medicine article recommends 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.