The 2009 flu pandemic is a global outbreak of a new strain of H1N1 influenza virus, often referred to as "swine flu". The virus, first described in April 2009, appears 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. The pandemic extends into 2010, although the number of cases are in steep decline.

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 the virus continued to spread globally, clinics in some areas were overwhelmed by people infected, and the World Health Organization (WHO) and US Centers for Disease Control (CDC) stopped counting cases and in June declared the outbreak to be a pandemic.

While only mild symptoms are experienced by the majority of people, some have more severe symptoms. Mild symptoms may include fever, sore throat, cough, headache, muscle or joint pains, and nausea, vomiting, or diarrhea. Those at risk of a more severe infection include: asthmatics, diabetics, those with obesity, heart disease, the immunocompromised, children with neurodevelopmental conditions, and pregnant women. In addition, even for persons previously very healthy, a small percentage of patients will develop viral 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.

H1N1 flu virus, even though it is often informally called swine flu, cannot be spread by eating pork or pork products. Similar to other influenza viruses, pandemic H1N1 is typically contracted by person to person transmission through respiratory droplets. Symptoms usually last 4–6 days. To avoid spreading the infection, it is recommended that those with symptoms stay home, away from school, work, and crowded places. Those with more severe symptoms or those in an at risk group may benefit from antivirals (oseltamivir or zanamivir). The CDC estimates that, in the United States alone, as of March 2010, there had been "about 12,000" deaths caused by swine flu. On January 18, 2010, Margaret Chan, WHO Director-General, said the pandemic appeared to be easing in the northern hemisphere but could still cause infections until winter ends in April, and that it was too soon to say what would happen once the southern hemisphere enters winter and the virus becomes more infectious. 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 and 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."