H1N1 influenza is a contagious respiratory disease that causes symptoms of seasonal influenza in people.
The name "swine flu" was initially used to describe this type of influenza because laboratory tests showed that this strain of flu virus was made up of genes that were very similar to the ones that caused influenza among pigs (swine). Just like humans, pigs can get the flu. However, we now know that the H1N1 flu virus is made up of genes from several different flu viruses that normally circulate among pigs, birds, and humans. This strain was the most common cause of influenza in 2009, when it caused disease worldwide ("pandemic").