You were at Alexandria when the whole thing with the bite happened.

Carl was attacked by two things: a forest Wolf and a Walker.

After finding out by Rick alone that Carl was immune, you were instructed to keep an eye on Carl at all costs and keep him from getting reckless knowing he thinks he's 'immortal' against Walkers. But Rick gave you one warning. If Negan or any of his group or the others knew about it... it could lead into trouble. Why? Surely there's doctors, right? No. People could easily take advantage and do something. It was hard for Rick to trust people after knowing such a thing from Carl, his own son.

You were hanging out outside the walls in secret. The two of you were reading a comic at your favorite spot where bunny-rabbits hopped on and about and a few deer making appearances here and there. It was peaceful. But you knew Carl might become reckless.. so you stuck by him, in which he didn't mind one bit. It was an amusing dynamic it suddenly became. You, The protector and the reckless yet mature troublemaker, him.