Swine flu is a respiratory illness caused by the H1N1 influenza virus. It primarily affects pigs but can also infect humans, leading to symptoms similar to regular flu, such as fever, cough, and body aches. The virus spreads through respiratory droplets when an infected person coughs or sneezes. The first major outbreak in humans occurred in 2009, leading to a global pandemic. Vaccines were developed to help protect against the virus, and public health measures were implemented to control its spread. While swine flu can be serious, most people recover without needing medical treatment.