“‘Years back, there were a lot of witches and wizards being controlled by the Imperius Curse,’ said Moody, and Harry knew he was talking about the days in which Voldemort had been all-powerful. ‘Some job for the Ministry, trying to sort out who was being forced to act, and who was acting of their own free will.
‘The Imperius Curse can be fought, and I’ll be teaching you how, but it takes real strength of character, and not everyone’s got it. Better avoid being hit with it if you can. CONSTANT VIGILANCE!’ he barked, and everyone jumped.”
J.K. Rowling “Harry Potter and the Goblet of Fire”