"Before the Wolfsbane Potion was discovered, however, I became a fully fledged monster once a month. It seemed impossible that I would be able to come to Hogwarts. Other parents weren't likely to want their children exposed to me. 

"But then Dumbledore became Headmaster, and he was sympathetic. He said that as long as we took certain precautions, there was no reason I shouldn't come to school...." Lupin sighed, and looked directly at Harry. "I told you, months ago, that the Whomping Willow was planted the year I came to Hogwarts. The truth is that it was planted because I came to Hogwarts. This house" -- Lupin looked miserably around the room, -- "the tunnel that leads to it -- they were built for my use. Once a month, I was smuggled out of the castle, into this place, to transform. The tree was placed at the tunnel mouth to stop anyone coming across me while I was dangerous."

 Harry couldn't see where this story was going, but he was listening raptly all the same. The only sound apart from Lupin's voice was Scabbers's frightened squeaking.

 "My transformations in those days were -- were terrible. It is very painful to turn into a werewolf. I was separated from humans to bite, so I bit and scratched myself instead. The villagers heard the noise and the screaming and thought they were hearing particularly violent spirits. Dumbledore encouraged the rumor.... Even now, when the house has been silent for years, the villagers don't dare approach it...." 

"But apart from my transformations, I was happier than I had ever been in my life. For the first time ever, I had friends, three great friends. Sirius Black... Peter Pettigrew... and, of course, your father, Harry -- James Potter."

 "Now, my three friends could hardly fail to notice that I disappeared once a month. I made up all sorts of stories. I told them my mother was ill, and that I had to go home to see her... I was terrified they would desert me the moment they found out what I was. But of course, they, like you, Hermione, worked out the truth...."

 "And they didn't desert me at all. Instead, they did something for me that would make my transformations not only bearable, but the best times