Her Sanguine Heart

Vampires can survive most things, even poisonous snake bites and absurd amounts of blood loss. It is a good thing, therefore, that Severus Snape is actually a vampire, thanks to a "prank," courtesy of Sirius Black. It is not something he has shared with many people, but extenuating circumstances force him to bare more than his love of blood to Hermione after she is attacked.