"Swine flu" was the popular name for the virus which was responsible for a global flu outbreak (called a pandemic) in 2009 to 2010. It's a type of seasonal flu and is now included in the annual flu vaccine.
The scientific name for swine flu is A/H1N1pdm09. It's often shortened to "H1N1".
The virus was first identified in Mexico in April 2009. It became known as swine flu because it's similar to flu viruses that affect pigs.
It spread rapidly from country to country because it was a new type of flu virus that few young people were immune to.
Overall, the outbreak was not as serious as originally predicted, largely because many older people were already immune to it. Most cases in the UK were relatively mild, although there were some serious cases.
The relatively small number of cases that led to serious illness or death were mostly in children and young adults – particularly those with underlying health problems – and pregnant women.
On 10 August 2010, the World Health Organization (WHO) declared the pandemic officially over.
The A/H1N1pdm09 virus is now one of the seasonal flu viruses that circulate each winter. If you've had flu in the last few years, there's a chance it was caused by this virus.
As many people now have some level of immunity to the A/H1N1pdm09 virus, it's much less of a concern than it was during 2009 to 2010.
The symptoms are the same as other types of common flu. They're usually mild and pass within 1 to 2 weeks. But as with all types of flu, some people are at higher risk of serious illness, particularly those with underlying health problems.
The regular flu jab will usually protect people who are at a higher risk of becoming severely ill. A vaccine programme for children has also been introduced, which aims to protect children and reduce their ability to infect others.
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