At the beginning of the season, the Prison was a safe haven for all, but minor threats from the inside put the whole group at risk. One of these minor threats came from Lizzie. Not comprehending that the walkers are dangerous, the tween was naming and “playing” with the zombies at the prison fence. While that initially seemed innocent and like a coping mechanism, things escalated when someone began feeding rats to the walkers, making them rowdy enough to start breaking down the fences.
Walking Dead viewers initially speculated that Lizzie was behind the act, but it wasn’t confirmed until episode 14. In the scene from the beginning of The Grove:
Carol’s frightened to discover that Lizzie is outside playing with a walker. Running outside to get Lizzie away from the zombie, Carol’s shocked to find Lizzie trying to protect it. But that doesn’t stop her from killing the walker.
And then a little later:
Carol and Tyreese come to a conclusion that they don’t need to go to Terminus – for now. But their home is not completely safe. Stationed near the train tracks, Lizzie heads back to feed a stuck walker. And when Mika follows her, all hell breaks loose. Mika discovers Lizzie feeding a rat to a walker, and when Mika tells her to stop playing pretend with the zombies, Lizzie completely snaps.
And even later in the same episode:
Later that night, Carol comes clean to Tyreese about killing Karen and David. “I had to stop the illness from breaking out,” she explains, sliding her gun to him. “I had to stop other people from dying … It wasn’t Lizzie. It wasn’t a stranger. Tyreese, it was me. Do what you have to do.”
Briefly, Lizzie feed rats to the walker, and Carol kills Karen and David.
Here's the official source an an article which is also relevant.
