A cough is something that you yourself can't help, but everybody else does on purpose just to torment you.
Source: You Can't Get There from Here, 'Can I Get You a Glass of Water? Or Please Close the Glottis After You'
-- Ogden Nash, (Aug 19 1902-1971), US poet; He was noted for off-beat poems like Candy is Dandy, But Liquor Is Quicker.