At the beginning of march, a flu that in many cases brought about respiratory problems, affected around 60% of the residents of a village in Mexico named La Gloria. This village is located near to a pig farm, which cultivates around one million of these animals each year. The owner of the pig farm declared that no clinical signals or symptoms which would suggest the presence of swine flu in the animals had been found. He also stated that none of the employees had shown any signs of illness either, and that all staff were routinely vaccinated.

The Mexican authorities attributed this increase to a late seasonal flu, until around the 21st of April, when the center for control and prevention of illness in the United States raised the alarm about two isolated cases of a new swine flu. The two first confirmed cases were children in the United States, a girl of nine years old in California and a boy of 10 years old in San Diego. These children became ill on the 28th and 30th of much respectively, having had no contact with pigs since travelling to Mexico. The first death due to swine flu occured on the 11th of April and was a girl who had been jailed since the 19th of March and was treated at the national institute of respiratory illness in Mexico. Following this, a woman who suffered from diabetes died on the 13th of April in the state of Oaxaca in Mexico. Samples were sent to the United States and Canada from Mexico on the 21st of April and these came back positive for swine flu. At this point the world health organization identified a new strain of the H1N1 flu.

The first cases of flu in Mexico, having been detected on the 11th of April, in the state of Veracruz, were followed by further cases in other states in Mexico, the United States and Canada. Soon after, cases began to appear in other countries where citizens had recently traveled to the affected countries. A few cases have also been counted where infection was passed on indirectly, and the patient had not visited another country. In march and april 2009, more than 1000 suspected cases of swine flu were reported in humans in Mexico and the south east of the United States. As such, there is no general agreement for the commonly seen question, how did the swine flu start.