Snape was not just one of the smartest students at Hogwarts, he was the smartest, at least in his own mind. So it was no great surprise that shortly after the hunger pains had ebbed and he came to realize that he was still sucking on a bloody washcloth, Snape compiled a list of his ailments and quickly placed his symptoms into neat little rows on a piece of parchment. Sensitivity to light, drinking blood, hours worth of time lost from memory the night before. The few things that his list pointed to made him frown. He hastily crossed off things like Chupacabra, Kappa, Sigbin and Lamia, he was not born one and they were not contagious. He brushed some of his greasy hair from his eyes and looked at the last word scrawled on his list. Vampire.
If he was completely honest with himself, he already knew it was the only feasible possibility, but he had hoped he could possibly just be ill with something else. After all, true vampirism would mean he would have to hand his wand over to the ministry. He wouldn't' t be counted as a wizard anymore, and if he so much as stepped out of line, the hunters would be set upon him. Life, if you could call it that, as a vampire was no life at all. However, so long as he still needed to breath, so long as he was still alive, he was technically only a half vampire, a Dampier.
With already one class missed that day, he decided to take a detour to the library for a bit of extra research and in light of his current situation he did see it as a waste of time. An hour and three books later he felt secure in the fact that as long as he didn't die, his 'condition' would not worsen and he would not have his wand revoked. It would be manageable and no one needed to know. The only problem he could foresee would be eating.
The blood craving would come again, though he didn't know when, and it wouldn't be long before his own juices would no longer satisfy him, but only make him hungrier. But the thought of drinking someone else's blood disturbed him. How much would he need to take? Who would he feed from? How would he get about without getting caught? Then again, Lupin had been sneaking out and running rampant since year one, surely he could as well, and if four people could make it through the halls undetected, Snape by himself should theoretically have an easy time of it.
It was not until weeks later that he had to test his sneaking skills. The blood lust had been growing slowly for days, but he had put off trying to eat for disgust of the meal and just so that he would know how long he could last. But by the end of that second night the cravings had become insatiable, it was almost all he could think about, so to save himself from simply biting the first person he saw, he left the dorms in search of a meal that would not be as vile as eating from a fellow Slytherin.
After so many years on friendly terms with Lily which ended only last year, he knew exactly where the Griffindor tower was. Naturally he couldn't just waltz in through the main portrait as he didn't know the current password, but as he could feel himself getting stronger these past few days, he decided to venture a climb. Getting outside from the dungeons was easy, but looking up at how far up the castle's high walls his new strength would have to take him, the remainder of his hunt would prove far more difficult. His better judgment told him it was a foolish venture, but his hunger drove him on.
Hogwarts was not a smooth and perfect castle and so he was able to find places to grip as he climbed. He had never been a strong person, in all honesty, Lily probably could have beaten him in an arm wrestling match, but as he climbed higher and higher he did not feel the familiar weakness burning in his muscles, and before he knew it he was face to glass with one of the windows that graced the Gryffindor dorms.
To his relief, the early school year nights were still warm enough to prompt the residents to keep their windows open to let in the fresh air. So, he carefully perched upon the wide window shelf and let himself in. If he had been a full vampire, he knew he would have needed permission to enter the private dormitories, but as it was, no such magic prevented him from silently picking out his first victim. Not wishing to rick waking any one by needlessly walking about, he settled on the bed closest to the window and slowly parted its canopy.
To his dismay, there behind the curtains lay the very recognizable sleeping form of James Potter. Snape almost left then to go find another room, but at the same time, he could hardly pass up the chance to hurt such a nemesis. He quietly cast 'petrificus', just in case James felt a need to awaken, then crept onto the bed, pulling the draperies closed behind him.
Under the effect of the spell, Potter's arms and legs became straight and stone stiff making him easy to roll over on his side and get at his neck. However, James had always been a light sleeper, and as he was rolled over his already open eyes looked straight at his attacker. Despite Snape's hunger, James' gaze startled Severus away from his prey causing him to fall ungracefully off the bed with James still frozen, humming in terror. The noise from Snape's hard landing on the floor and Potter's muffled screams woke some of the others in the small bedroom chamber. Severus could hear them rising from their beds to see what was the matter, and in a moment of panic, Severus untangled himself from the torn canopy and jumped out of the open tower window.
Snape awoke two days later in the hospital wing, sore and hungry, so very hungry. The room should have been too dark for him to see properly but the bits of moonlight filtering in through the tall windows perfectly outlined the rows and rows of beds. Each one depressingly empty other than his and a small sleeping form on the bed closest to the door. The only thought in his whole mind was that he needed to eat. He slid from his bed and padded barefooted to the sleeping meal.
Lupin, as sickly looking as ever, slept restlessly on his cot unaware of the danger he was in. Uncaring if he was recognized now or not, Severus pounced on the sleeping figure with the full brunt of his strength, and as his human teeth sank painfully into Lupin's neck, the young werewolf awoke screaming in agony.