Swine flu is caused by a virus similar to the type of flu virus that, in various forms, infects people every year, but is a strain typically found only in pigs -- or in people who have direct contact with pigs.

CNN: Among the swine flu mysteries: Why only deaths in Mexico?

The last attempt to inoculate the U.S. population against a type of swine flu occurred in 1976 after some 200 soldiers from Fort Dix, New Jersey, became infected.

CNN: Shipments of H1N1 flu vaccine leave factories

Jennifer Scott was transferred by helicopter from Dumfries and Galloway Royal Infirmary after she was diagnosed with a serious "flu-type illness".

BBC: Scottish flu victim airlifted to Leicester hospital

Swine flu is a respiratory disease of pigs caused by type A influenza, according to the CDC.

CNN: CDC confirms 7 cases of swine flu in humans

This was a second type of influenza: pandemic, rather than seasonal, flu.

ECONOMIST: Flu vaccination

Swine flu is a respiratory disease which is caused by a strain of the influenza type A virus known as H1N1.

BBC: India swine flu outbreak 'kills 94'