Swine flu is an infection caused by a virus. It's named for a virus that pigs can get. People do not normally get swine flu, but human infections can and do happen. In 2009 a strain of swine flu called H1N1 infected many people around the world.
The virus is contagious and can spread from human to human. Symptoms of swine flu in people are similar to the symptoms of regular human flu and include fever, cough, sore throat, body aches, headache, chills and fatigue. Source: MedlinePlus
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. On June 11, as the virus continued to spread on several continents, WHO declared the outbreak to be an influenza pandemic (Phase 6).
Officials now believe the outbreak of the new flu strain began in Mexico in March 2009, or perhaps earlier. The novel “H1N1 swine flu” was first identified in California in late April. Health officials quickly linked the new virus to many of the illnesses in Mexico. Since then, cases have been reported around the world. Source: The 2009 Influenza Pandemic: An Overview, Congressional Research Service