On June 11, 2009, in response to the global spread of a new strain of H1N1 influenza ("flu"), the World Health Organization (WHO) declared the outbreak to be an influenza pandemic, the first since 1968. WHO said that the pandemic declaration was based on the geographic spread of the new virus, not on increasing severity of the illnesses it causes. Officials now believe the outbreak began in Mexico in March, or perhaps earlier. The novel "H1N1 swine flu" was first identified in California in late April. Health officials quickly linked the new virus to many of the illnesses in Mexico.

In March 2009, a number of cases of an influenza-like illness and severe respiratory infections in humans were reported in parts of Mexico. These cases were later confirmed to be a strain of influenza A(H1N1), commonly referred to as swine flu and later called 2009 H1N1. By the end of April 2009, confirmed human cases of 2009 H1N1 infection were reported throughout Mexico, in parts of the United States, and in several countries worldwide. Reports of the outbreakcoupled with the use of the initial moniker swine fluinitially caused a downturn in domestic and international pork markets.

On April 29, 2009, the World Health Organization (WHO) raised its influenza (flu) pandemic alert level to Phase 5, one level below declaring that a global influenza pandemic was underway. According to WHO, the declaration of Phase 5 is a strong signal that a pandemic is imminent and that the time to finalize the organization, communication, and implementation of the planned mitigation measures is short. The WHO has increased the pandemic flu alert level in response to the spread of a new strain of influenza A subtype H1N1 virus.