As of April 27, 2009, the Government of Mexico has reported 18 laboratory confirmed human cases of swine influenza A/H1N1 infection, according to the Centers for Disease Control. Investigation is continuing to clarify the spread and severity of the disease in Mexico. Suspect clinical cases have been reported in 19 of the country's 32 states. The World Health Organization (WHO), the Global Alert and Response Network (GOARN), and the Centers for Disease Control and Prevention (CDC) have sent experts to Mexico to work with health authorities. CDC has confirmed that seven of 14 respiratory specimens sent to CDC by the Mexican National Influenza Center are positive for swine influenza virus and are similar to the swine influenza viruses recently identified in the United States.

On April 25, the WHO Director-General declared this event a Public Health Emergency of International Concern under the rules of the International Health Regulations. CDC and state public and animal health authorities are currently investigating 20 cases of swine flu in humans in California, Texas, Kansas, Ohio, and New York City. Some of the U.S. cases have been linked to travel to Mexico. CDC is concerned that continued travel by U.S. travelers to Mexico presents a serious risk for further outbreaks of swine flu in the United States.

The CDC also reported the following swine flu incidents in the U.S.: California: 7 cases; Kansas: 2 cases; New York City: 28 cases; Ohio: 1 case; and Texas: 2 cases. Of these 40 cases, the CDC said, there have been zero deaths. Only one has been hospitalized. All 40 have fully recovered.