The term "swine flu" refers to a strain of influenza that primarily affects pigs but can also infect humans. It is caused by the H1N1 virus, which was first identified in 2009 as a new strain of swine flu and quickly became a global pandemic. The name "swine flu" comes from the fact that the virus typically spreads among pigs, but it can also be transmitted between humans, leading to outbreaks like the one in 2009.