The 2009 flu pandemic is a global outbreak of a new strain of H1N1 influenza virus, often referred to colloquially as "swine flu". Although the virus, first detected in April 2009, contains a combination of genes from swine, avian (bird), and human influenza viruses, it cannot be spread by eating pork or pork products. 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, 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. 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 USA alone, and as of November 14, there had been 9,820 deaths (range 7,070-13,930) 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.

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