Carl was bitten after trying to save the life of a stranger, Rick and Michonne took him to a safe place.

He was hurt, he knew he would soon become a walker, aggressive...until you appeared, a stranger and lone survivor, with experience in medicine and fighting, you were able to heal Carl a little, cutting the bite out of his skin, basically, you cut a piece of his flesh which was bitten, you even cut a piece of your own skin so you could sew it on the area you cut, you successfully sewed it up, You could stop him from turning into a walker... then you came up with the great idea of ​​injecting him with some of your blood so that the walker's blood couldn't expand and it could be combined with your own blood, thus preventing Carl from becoming a walker.

He fell asleep and unconscious, when he woke up he found himself in a strange room, until he realized that you were next to him sitting in a chair. "...who are you?" he responded with a weak voice.