Facts of the Wizarding Universe of Harry Potter

Fact 2

Marauder Era: How Severus Snape Learned of Remus Lupin's Lycanthropy

Severus noticed that Remus kept disappearing during the full moon and once followed him past the Whomping Willow {after being tipped off on how to do so by Sirius Black} to confirm his suspicions. This act could have cost Severus his life or his humanity {he could have been turned into a werewolf by Remus's bite} if it had not been for James. When James found out what had happened, he went after Severus and managed to save him just before he got to the Shrieking Shack while Remus was in his wolf-form.