Swine flu is caused by a virus. The most common subtype, or strain, is influenza type A H1N1, and this subtype has also caused infection in people; however, this H1N1 swine flu virus is different from the H1N1 human virus. The letters H and N in the subtype name stand for proteins found on the surface of the virus, which are used to distinguish between different subtypes. Other virus subtypes found among pigs include H1N2, H3N1, and H3N2.
Influenza viruses are constantly changing their genes, a process called mutation. When a swine flu virus is found in humans, it is said to have "jumped the species barrier." This means that the virus has mutated in a way that allows it to cause the condition in humans. Because humans have no natural protection or immunity to the virus, they are likely to become ill.
Pigs can be infected by swine influenza, human influenza viruses, and avian (bird) influenza viruses. When a pig is infected with influenza viruses from different species, the viruses can swap genes and a new virus that is made up of genes from the various viruses from different species can appear.
Humans do not normally become infected with swine flu. However, there have been periodic human infections; most of these cases occur in people with direct exposure to pigs (e.g., people working on pig farms or slaughterhouses). People have also infected pigs with strains of human flu virus as well.
Person-to-person transmission of swine flu has been documented, but it is not clear how easily the virus is spread among people. It is believed that the virus spreads the same way as regular seasonal influenza. It is spread from person to person when the virus enters the body through the eyes, nose, and/or mouth when coughing and sneezing. The virus can also rest on hard surfaces like doorknobs, ATM buttons, and counters. A person who touches these surfaces with their hands and then touches their eyes, mouth, or nose can become infected with the virus.
You cannot get infected with swine flu from eating pork products that have been properly cooked to an internal temperature of 71°C (160°F).