The swine flu epidemic of 2009 began in La Gloria, Veracruz, Mexico in March. Sixty percent of the residents got sick and two infants died in the outbreak. No testing was done on the infants to determine their cause of death. The government thinks that H3N2 influenza is the cause of this outbreak even though one of the residents previously tested positive for H1N1. A 9-year old girl living in California gets sick at the end of March 2009. A 10-year old boy in San Diego, California develops flu-like symptoms. Testing of nasopharyngeal swabs is done several times and swine influenza finally confirmed for both children in April.

In the beginning of April, a 4 year old boy in Veracruz falls ill. His sample is sent out of the country for testing and H1N1 is confirmed. Shortly thereafter, the European Union is alerted about the outbreak in Mexico via MedlSys. Meanwhile public health authorities in Mexico are investigating unusual cases of pneumonia. Some people develop acute respiratory distress syndrome in which they have increased difficulty breathing 3 to 6 days after their symptoms began. A 39 year old woman in San Luis Potosi, Mexico dies as does an individual in Oaxaca, Mexico. Both are later confirmed to be due to swine flu.

National surveillance in Mexico is stepped up because of the atypical pneumonia in Oaxaca. Mexico sends samples from individuals showing severe symptoms to the Centers for Disease Control in the U.S. to be tested. On April 21, the CDC alerts doctors to the new strain of influenza that was found in the nine year old girl and ten year old boy. Surveillance is enhanced and the Associated Press covers the alert by the CDC, mentioning H1N1 for the first time in English-speaking media.