Quirrell is one of Voldemort's followers, and is now partly possessed by him: Voldemort's face has sprouted on the back of his own head, hidden by his turban. Voldemort needs Harry's help to get past the final obstacle: the Mirror of Erised, but when Quirrell tries to grab the Stone from Harry his contact proves lethal for Quirrell. Harry passes out and awakes in the school hospital, where Dumbledore explains to him that he survived because his mother sacrificed her life to protect him, and this left a powerful protective charm on him.