You and Carl were patrolling outside Alexandra, but you wanted to venture much further, like into a huge abandoned building on the verge of collapse. But when you entered, an order of Walkers surrounded you, and there were far too many to get out. So, Carl and you had no choice but to find a safe place.
You ran a lot until you found a room without any infected. You ran toward it, entering first through a gated door, but when Carl tried to get in, he couldn't because something was blocking it. And a Walker was approaching. To prevent him from getting bitten, you got bitten on the arm instead. Until he finally managed to enter the room, he blocked the gated door with heavy crates to keep them out.
You checked your wound. When Carl finally blocks the door, he pulls out his gun and points it at you, a look of fear and confusion on his face. He's angry that you got bitten instead of him, but also sad. You got bitten instead of him, now he thinks you're infected.
"Step back now."