The 2009 swine flu outbreak

The 2009 swine flu 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 influenza-like illness were first detected in three areas of Mexico, but the new strain was not clinically ascertained as such until a month later in cases 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., the Northern Hemisphere were also identified. By April 27, the new strain was confirmed in Canada, Spain, and the United Kingdom and suspected in many other nations, including New Zealand, with over 2,400 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, including a strain endemic in humans and two strains endemic in pigs, as well as an avian influenza.

In April both the WHO and the United States Centers for Disease Control and Prevention (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 26, 2009, Mexico City schools, universities, and all public events remained closed or suspended while other schools in the U.S. closed due to confirmed cases in students.
On April 27, 2009, Mexican government officials announced a nationwide shut down of schools.

Swine influenza (also swine flu) refers to influenza caused by any virus of the family Orthomyxoviridae, that is endemic to pig (swine) populations. Strains endemic in swine are called swine influenza virus (SIV), and all known strains of SIV are classified as Influenzavirus A (common) or Influenzavirus C (rare).
Influenzavirus B has not been reported in swine. All three clades, Influenzavirus A, B, and C, are endemic in humans.

People who work with poultry and swine, especially people with intense exposures, are at risk of infection from these animals if the animals carry a strain that is also able to infect humans. SIV can mutate into a form that allows it to pass from human to human. The strain responsible for the 2009 swine flu outbreak is believed to have undergone this mutation.

In humans, the symptoms of swine flu are similar to those of influenza and of influenza-like illness in general.