"Severus is loyal," Lucius began, "because he is Dark. Not just a Dark Wizard, but a Dark Creature."
Harry's eyes widened and he leaned forward. This was news to him. "Explain."
"When Severus was nineteen, he was attacked by a vampire."
"Yes," Harry interrupted. "He managed to kill it and escape." The vampire had been the child of one Harry had later worked with during the war. There had been quite a bit of tension between Severus and the vampire and Harry had broken up several, rather vicious, fights between them.
"Not before being bitten," Lucius continued. "It attempted to turn him, but the moment Severus regained enough strength, he killed it. One of our house elves found him unconscious on the doorstep the next morning. Narcissa is quite skilled at healing and tried her best to save him, but the vampirism had already progressed too far. We had no choice but to send him to Dumbledore at Hogwarts. Poppy Pomfrey is a licensed mediwitch. She had access to potions and knowledge of spells that we didn't. She did her best to halt the change and eventually succeeded. Severus is still a wizard, but I would say, he's a little more vampire than human. He needs blood regularly, but is only slightly irritated by the sun. He seems to be mortal, but has a longer lifespan. He's only aged a few years since that day.
"He's able to pass for human and only a handful of people know, including myself, Narcissa, Draco, Dumbledore and the Hogwarts staff. Dumbledore likes to think that Severus is loyal to him because he could out him, much as he did Lupin. Severus would be hunted down and killed because, not only is he a vampire, he is also a convicted Death Eater. Dumbledore counts on his fear of discovery to override his natural loyalty to the Dark."
The Curse of Fate, Chapter 17