Table 1.
Timeline of swine-origin H1N1 virus outbreak (see also 32)
| Mid-February | Outbreak of respiratory illness in La Gloria, Veracruz, Mexico31 |
| April 12 | Mexican public health authorities report outbreak in Veracruz to the PAHO |
| April 15 | CDC identifies S-OIV in the specimen of a boy from San Diego, California |
| April 17 | CDC identifies S-OIV in the specimen of a girl from Imperial, California |
| April 21 | CDC alerts doctors to a novel strain of H1N1 influenza virus |
| April 23 | The Public Health Agency of Canada identifies S-OIV in specimens from Mexico |
| April 24 | WHO issues Disease Outbreak Notice |
| April 27 | International spread and clusters of human-to-human transmission prompt WHO to raise the pandemic alert from phase 3 to 4 |
| April 29 | WHO raises the pandemic alert from phase 4 to 5 (human-to-human spread in at least two countries in one WHO region) |
| May 21 | 41 countries report 11,034 cases, including 85 deaths |