do you guys think the illness in the prison were spread by rats or something? I was suspicious of those two little (now orphaned) girls who were naming the zombies, and later rick or someone found dead rat carcasses on the side of the fence and they said someone was feeding the zombies.. and man that piglet scene was
it look so real and rick's actor is so good at his emotions 
What was found in the prison moments before the main event left an ambiguous clue, the illness spreading by rats is one possiblity but the other, we never found out who was feeding the walkers. In the episodes leading up to the mid season finale, there was something else, basically people suspect that a 3rd party exists and caused some chaos, we just don't know yet. We saw some horrid things in the forest and even somthing the other camp was vulnerable too, some group looted and shot another group offscreen, this is something that many suspect to be a major factor into the next half of the season.
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