Initial outbreaks

Initial outbreaks
Further information: 2009 swine flu outbreak timeline

First diagnosis
The new strain of swine flu was first diagnosed as such when the CDC received a sample on April 14 from a patient who fell ill on March 30 in San Diego County, California. A second case was confirmed on April 17 who had fallen ill on March 28 in Imperial County, California.[60][61]


Spread within Mexico
Further information: 2009 swine flu outbreak in Mexico
The outbreak was first detected in Mexico City, where surveillance began picking up a surge in cases of influenza-like illness (ILI) starting March 18.[62] The surge was assumed by Mexican authorities to be "late-season flu" (which usually coincides with a mild Influenzavirus B peak)[63] until April 21,[64][65] when a U.S. Centers for Disease Control and Prevention alert concerning two isolated cases of a novel swine flu was reported in the media.[66] Some samples were sent to the U.S.-based CDC on April 18.[67] The Mexican cases were confirmed by the CDC and the World Health Organization to be a new strain of H1N1.[62][68]

Cases were also reported in the states of San Luis Potosí, Hidalgo, Querétaro and Mexico State.[69] Mexican Health Minister José Ángel Córdova on April 24, said "We’re dealing with a new flu virus that constitutes a respiratory epidemic that so far is controllable."[69] Mexican news media report that the outbreak may have started in February near a Smithfield Foods pig plant amid complaints about its intensive farming practices,[70][71] although no pigs in Mexico have tested positive for the virus.

The first death from swine flu occurred on April 13, when a diabetic woman from Oaxaca died from respiratory complications.[72][73] The Mexican fatalities are alleged to be mainly young adults of 25 to 45, a common trait of pandemic flu.[54][74] Although there have been reports of 152 "probable deaths"[75] in Mexico City and "more than 100 dead from swine flu",[76] the WHO had received reports of only 16 confirmed deaths total and explicitly denied the larger figure as of April 29.[77][78]


Seasonal variation
The high humidity of summer and the increase in exposure to ultraviolet light typically leads to the end of the flu season as the virus becomes less likely to spread.[79] In Mexico City, May marks the end of the dry season, and experts speculate that the spread of the swine flu may slow.[80] On April 30 Mexican Health Secretary Jose Angel Cordova voiced cautious optimism that the spread of the swine flu might be slowing in that country.[81] However, the outbreak comes at the beginning of the flu season for Southern Hemisphere countries such as New Zealand, Australia, South Africa, and parts of South America