A Tale of Immortality
Quirinus Quirrell had taken a sabbatical from Hogwarts in order to conduct research on vampires. In reality he was conducting research on immortality. Quirinus had contracted a fatal illness and the only way to prolong his "life", so to speak, was to become a non-living, but near immortal vampire.
Quirrell had come to Albania to conduct his field research as a large population of vampires resided there. Unfortunately, vampires more frequently drained their victims rather than turn them and Quirrell had been forced to kill several vampires in self defense. This did nothing to increase the likelihood of any vampire willingly turning Quirrell into one of their own.
One evening Gilderoy Lockhart "chanced" upon his fellow Ravenclaw at a roadside inn. He and Quirrell talked long into the night. The next morning an obliviated Quirrell and a cocky Lockhart parted ways.
Gilderoy retreated to the luxury and safety of Italy where he spent the next few months writing his next best seller, Voyages with Vampires.
Quirrell, continued on his quest to become a vampire with no memory of the vampires he had already killed.
Unbeknownst to all, Quirrell was a parselmouth. It is not wise to disclose such information to others. While searching for a vampire he chanced upon a snake and the two began to talk...
The talked turned to plotting...
The plotting turned into a plan.
Quirrell exited the woods, embodied by a second soul and with a new goal in mind: immortality via the philosophers stone.