"On April 29, 2009, in response to the global spread of a new strain of influenza, 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.' Officials now believe the outbreak began in Mexico in March 2009, or perhaps earlier. The novel flu virus was first identified in California in late April 2009. Health officials quickly linked the new flu strain to many of the illnesses in Mexico. Since then, growing numbers of cases have been reported around the world. As of May 20, 2009, more than 10,000 cases had been reported in 41 countries in North and South America, Europe, Asia, and Oceania. Most of the reported cases are in Mexico, the United States, and Canada. Health officials note that reported cases likely represent only a fraction of actual infections. For example, a U.S. official commented that there may actually have been upwards of 100,000 cases thus far in the United States."
CRS Report for Congress, R40554