During March, Mexico saw unusual patterns of acute cases of respiratory infection. On 18 April, a laboratory in the United States reported two human cases of swine flu—the result of a novel reassortment of influenza A strain H1N1 from avian, swine, and human strains—in two children from California. A week later, on 25 April, the World Health Organization declared the swine flu outbreak in North America a “public health emergency of international concern.” This decision, in accordance with the International Health Regulations, means that countries have been asked to step up reporting and surveillance of the deaths and illnesses associated with the disease. On 29 April, the International Health Regulations emergency committee recommended a change from WHO pandemic influenza phase 4 to phase 5. This means that WHO views a pandemic as imminent.

From 17 April to 26 April, 1840 suspected cases of flu with severe pneumonia were reported in Mexico, 26 of which have been confirmed as swine flu. More than 150 people have died, and many have been in the 20-40 year age range. As of …