The Plague was a dreadful disease which swept England, killing thousands and thousands of people. It was also known as the Black Death because just before they died they turned completely green. (At this time the colour 'green' had not yet been given its name, and any colour that people were unsure of, they called 'black'. Hence the Black Knight was actually bright yellow, the Black Mountains were purple, and Black Magic was relly a funny sort of orange colour.) The cause of the play remains a mystery. One theory was that it was brought to England by rats who lived on ships. Another theory is that it was cause by a population eating too much rhubarb, which caused large numbers of people to swell up and burst. A third theory is that it was a germ that came from Outer Space and infected a whole country. Another theory is that all these theories are just guesswork because all the doctors died in The Plague, so there was no one left who knew for certain what had caused it.