2009 swine flu pandemic in Mexico

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In March and April 2009, an outbreak of a new strain of influenza commonly referred to as "swine flu" infected many people in Mexico and other parts of the world, causing illness ranging from mild to severe. Initial reports suggested that the outbreak had started in February due to farming practices at a pig farm half-owned by Smithfield Foods.^[4] Smithfield Foods stated that it had found no clinical signs or symptoms of the presence of swine influenza in the company's swine herd, or among its employees at its joint ventures in Mexico, that it routinely administers influenza virus vaccination to their swine herds and that it conducts monthly testing for the presence of swine influenza.^[5] The new strain was identified as a combination of several different strains of Influenzavirus A, subtype H1N1, including separate strains of this subtype circulating in humans and in pigs.

The World Health Organization (WHO) and the U.S. Centers for Disease Control and Prevention (CDC)^[6] expressed serious concerns that the new strain, which transmits between humans and has had a relatively high mortality rate in the possible and confirmed Mexican cases, has the potential to become an influenza pandemic. It was reported that, because the virus was already widespread, containment would be impossible.^[7] The WHO declared a public health emergency of international concern in response to the outbreak on April 25, 2009.^[8]

As of April 27, 2009, all schools nationwide remained closed until at least May 6,^[9][10] with "non-essential" businesses ordered to close as of April 30.^[11] Health Minister Córdova noted that the number of new cases had declined during the three days: from 141 on Saturday to 119 on Sunday and 110 on Monday.^[12] It is unclear why there were more deaths in Mexico than in other areas, as there were multiple potential variables, such as a stronger strain of the virus or more exposure to it.^[13] No definite conclusion had been reached, however the CDC reported that swine flu viruses in the US and Mexico matched.^[14]