You have flitted into Lord Voldemort’s mind without damage to yourself, but he cannot possess you with-out enduring mortal agony, as he discovered in the Ministry.
Last time he had fallen off his broom due to the presence of dementors around the pitch, and the time before that, all the bones had been removed from his arm by the incurably inept Professor Lockhart…… That had been his most painful injury by far … he remembered the agony of regrowing an armful of bones in one night, a discomfort not eased by the arrival of an unexpected visitor in the middle of the — Harry sat bolt upright, his heart pounding, his bandage turban askew.
These pictures" — he indicated a few of them as he swept past — "give a fair representation of what happens to those who suffer, for instance, the Cruciatus Curse" — he waved a hand toward a witch who was clearly shrieking in agony — "feel the Dementor’s Kiss" — a wizard lying huddled and blank-eyed, slumped against a wall — "or provoke the aggression of the Inferius" — a bloody mass upon ground.
Someone was screaming, he would surely die of this agony, Snape was going to torture him to death or madness — "No!" roared Snape’s voice and the pain stopped as suddenly as it had started; Harry lay curled on the dark grass, clutching his wand and panting; somewhere overhead Snape was shouting, "Have you forgotten our orders?