Swine flu is the popular name for influenza (flu) caused by a relatively new strain of influenza virus A. It was responsible for the flu pandemic in 2009-10.
The virus is officially known as influenza virus A/H1N1pdm09.
The virus was first identified in Mexico in April 2009 and was also known as Mexican flu. It became known as swine flu because the virus closely resembled known influenza viruses that cause illness in pigs.
It spread rapidly from country to country because it was a new type of flu virus that few people were immune to.