The 2009 flu pandemic was a global outbreak of a new strain of H1N1 influenza virus, often referred to as "swine flu" or "Mexican flu". First described in April 2009, the virus appeared to be a new strain of H1N1 which resulted when a previous triple reassortment of bird, pig and human flu viruses further combined with a Eurasian pig flu virus (see figure 1). 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 persons, a small percentage will develop pneumonia or acute respiratory distress syndrome. 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.

Figure 1. Schematic representation of the 2009 triple reassortant influenza virus.

The outbreak began in the state of Veracruz, Mexico, with evidence that there had been an ongoing epidemic for months before it was officially recognized as such. The Mexican government closed most of Mexico City's public and private facilities in an attempt to contain the spread of the virus; however, it continued to spread globally, and clinics in some areas were overwhelmed by infected people. In June, the World Health Organization (WHO) and US Centers for Disease Control (CDC) stopped counting cases and declared the outbreak a pandemic.

Despite being informally called "swine flu", the H1N1 flu virus cannot be spread by eating pork or pork products; similar to other influenza viruses, it is typically contracted by person to person transmission through respiratory droplets. Symptoms usually last 4–6 days. Antivirals (oseltamivir or zanamivir) were recommended for those with more severe symptoms or those in an at-risk group. Currently, there are 14,286 confirmed deaths worldwide. This figure is a sum of confirmed deaths reported by national authorities; the WHO states that total mortality (including deaths unconfirmed or unreported) from the new H1N1 strain is "unquestionably higher".

The pandemic began to taper off in November 2009, and by May 2010, the number of cases was in steep decline. On 10 August 2010, the Director-General of the World Health Organization announced the end of the H1N1 pandemic. The WHO noted that the H1N1 pandemic could have been much worse. According to the latest WHO statistics, the virus has killed more than 18,000 people since it appeared in April 2009, approximately 4% of the 250,000 to 500,000 annual influenza deaths. Research released in September 2010 disclosed that children with the pandemic flu were less likely to develop complications than those sick with seasonal flu strains, contradicting early reports on the severity of the pandemic. Critics claimed the WHO had exaggerated the danger, spreading "fear and confusion" rather than "immediate information". The WHO began an investigation to determine whether it had "frightened people unnecessarily".