Swine Flu Outbreak

The 2009 swine flu outbreak or H1N1 outbreak is the spread of a new strain of H1N1 influenza virus that was first detected by public health agencies in March 2009. Local outbreaks of an influenza-like illness were first detected in three areas of Mexico, but the new strain was not clinically identified as such until a month later in Texas and California, whereupon its presence was swiftly confirmed in various Mexican states and Mexico City; within days isolated cases elsewhere in Mexico, the U.S., and several other Northern Hemisphere countries were also identified. By April 28, the new strain was confirmed in Canada, Spain, the United Kingdom, New Zealand and Israel and suspected in many other nations, including South Korea and Austria, with over 2,500 candidate cases, prompting the World Health Organization (WHO) to raise their pandemic alert level to 4. A level 4 warning officially means that the WHO considers that there is "sustained human to human transmission"; whereas levels 5 and 6 represent "widespread human infection".

The new strain is an apparent reassortment of several strains of influenza A virus subtype H1N1, which analysis at the United States Centers for Disease Control and Prevention (CDC) identified as a strain endemic in humans, a strain endemic in birds, and two strains endemic in American and Eurasian pigs (swine).
In April both the WHO and the CDC expressed serious concerns about the situation. It had the potential to become a flu pandemic because the strain was novel, transmitted from human to human against little immunity, and the Mexican mortality rate was unusually high. On April 25, 2009, the WHO determined the situation to be a formal "public health emergency of international concern", with knowledge lacking in regard to "the clinical features, epidemiology, and virology of reported cases and the appropriate responses". Government health agencies around the world also expressed concerns over the outbreak and are monitoring the situation closely.

As of April 28, 2009, Mexico's schools, universities, and all public events remained closed or suspended while other schools in the U.S. closed due to confirmed cases in students.