Quirrell is, in effect, turned into a temporary Horcrux by Voldemort. He is greatly depleted by the physical strain of fighting the far stronger, evil soul inside him.
Quirrell was wandering through the Black forest when he met a nasty bit Voldemort and he agreed to become a death eater. Quirrell couldn't steal the sorcerer's stone so Voldemort attached himself to Quirrell.
Quirinus Quirrell served as a temporary Horcrux when Voldemort's soul possessed his body in 1991. However, the piece of the soul survived Quirrell's demise, and was destroyed six years later when Voldemort was killed.
Near the end of his life, his reason for wearing the turban became clear. Quirrell had joined with Voldemort and agreed to allow Voldemort's fledgling body to latch on to the back of his head. It was only able to grow into a face, and used Quirrell for transportation and sustenance.
Quirrell, curiously, is apparently the only evil character that utters the name "Voldemort", though he does it ("at least", so to say) with the respectful "Lord" before it.
Voldemort's desire to beat death and become immortal was also why he and Quirrell ended up slaying innocent unicorns. Drinking the blood of a unicorn would keep someone alive – even if they were an inch from death. However, it is considered an act of evil to kill something so pure.
Harry makes use of two of the Unforgivable Curses in the books. Dueling with Bellatrix Lestrange, he attempts the Cruciatus curse, with limited results; Bellatrix says that he has to really hate someone to make the Unforgivable Curses work properly, righteous indignation isn't enough.
Voldemort intentionally made six Horcruxes, but when he used Avada Kedavra on Harry, he unintentionally created a seventh Horcrux. Instead of dying, Lily's love for Harry created a counter 'curse' known as Sacrificial Protection and saved Harry.
Dec. 28, 1997: Ron Weasley destroys Salazar Slytherin's locket deep in the Forest of Dean, Gloucestershire, by stabbing it with Godric Gryffindor's sword.
At the time Harry touched him, Quirrell was sharing a body with Voldemort. Since the magical blood protection Harry had prevented Voldemort from touching him, Quirrell would suffer pain. He would ultimately be destroyed from touching Harry.
That worked because when Lily was killed, she had tried to protect Harry and therefore gave him the most powerful weapon against Voldemort; Love. Love is the one thing that Voldemort is unable to handle and therefore when Quirrell was touched by Harry he burnt him with love.
The Dark Lord — aka Voldemort — created seven Horcruxes in an attempt to sustain immortality.
Snape's explanation to the Death Eaters was that he knew nothing of Voldemort's involvement, but "saw only greedy and unworthy Quirrell attempting to steal the stone." It was confirmed in Deathly Hallows that Snape had been "keeping an eye" on Quirrell on Dumbledore's orders.
TL; DR - Dumbledore knew by Quirrell's behavior that he was possessed by Voldemort. He moves Quirrell to Defense Against the Dark Arts to remove him (and Voldemort) from the school by the end of the year.
So Quirrell was possessed by Lord Voldemort for around two years. How did Voldemort meet Quirrell in Harry Potter? Quirrell was wandering through the Black forest when he met a nasty bit Voldemort and he agreed to become a death eater.
Narcissa's greatest moment, however, was when she chose to betray Voldemort in Harry Potter and the Deathly Hallows. Not knowing whether Draco was alive or dead, Narcissa chose to lie to the Dark Lord rather than risk losing her son.
Phineas Nigellus's portrait inside it overheard Hermione and reported that to Snape. Snape thus knows that Harry and Hermione are in the Forest of Dean, though not exactly where; the Forest of Dean is a largish place, several wooded sections totaling about 40 square miles.
Narcissa Malfoy's Very Dangerous Lie
But she was also a mother, which meant she was willing to risk everything to make sure her son was safe. When Harry survived Voldemort's Killing Curse for the second time, Narcissa pretended he was dead so she could get to Draco.
The Killing Curse was created in the early Middle Ages by Dark witches and wizards. Primarily, the curse was used to quickly slay opponents during a duel. In 1707, the Wizards' Council was restructured into the Ministry of Magic, which allowed for more strict restrictions on certain types of magic.
Cruciatus Curse ('Crucio')
Harry tried to use this spell himself. The first time was against Bellatrix after she had killed Sirius. His attempt was unsuccessful because, as Bellatrix mockingly explained: 'You need to really want to cause pain – to enjoy it – righteous anger won't hurt me for long…'
Dumbledore, at one point, possessed all three Deathly Hallows. From Dumbledore's letters to Grindelwald in Deathly Hallows, it is obvious that the headmaster of Hogwarts was obsessed with the idea of the Hallows in his youth.
When Harry first meets Quirrell, he has adopted a turban for everyday wear. His nerves, expressed most obviously in his stammer, are so pronounced that it is rumoured the turban is stuffed full of garlic, to ward off vampires.
When they reach Fluffy, Harry, Hermione, and Ron notice a harp by his feet and realize that someone has already passed by Fluffy. Harry plays a flute he has brought, putting Fluffy to sleep and allowing his gang to go through the trapdoor.
In typical wizarding fashion, both meanings of "two-faced" are meant in referring to Quirrell: he has been two-faced in that he has been deceiving people and leading a double life, and he has been leading a double life literally, complete with two faces, since his failure to steal the Stone from Gringotts.