Bob wonders if this came from the walkers, but Dr. S. says stuff like this was around first. As for why the cell door was locked, Daryl explains that the guy used to sleepwalk. That seems like a trait that the zombie apocalypse would weed out of the population right away for several obvious reasons, but there you go. "How could somebody die in a day just from a cold?" Daryl wonders. I don't know about the others, but maybe Patrick got it from shaking hands with someone who had just licked his fingers, Daryl. Rick says the same thing happened to one of his pigs, plus the boar in the woods. Hershel says pigs and birds are standard vectors. "We need to do something about those hogs." Bob adds that contagion spreads among humans as well, in close quarters. "Doesn't get any closer than this." Hershel is the one to pronounce the bottom line: "All of us in here, we've all been exposed."
Carol leads the two girls, Lizzie and Mica, into the cell with their dad. Lizzie's the older one of the two, the one Carl caught calling walkers by name last week. Ryan's last words are to tell her to take care of her sister and when he expires forthwith, Carol starts to shoo the girls out so she can take care of the necessary follow-up. But when she draws her knife, Lizzie says she should do it. Carol lets her have the knife and tells her to get to it quickly. "Remember what I showed you." But Lizzie loses her nerve, and ends up getting comfort from her little sister, who tells her to look at the flowers on the shelf while Carol almost gently slides her knife through Ryan's temple. And that's the way they all became the Carol bunch.
Carl and Maggie are slowly and painfully helping Michonne back to the prison, until Carl sees Rick coming and runs into his arms, ignoring Rick's warning to stay back. Rick comforts him a bit before making him step back and Carl immediately confesses to using one of the guns. Michonne explains what happened at the gate and Maggie asks what happened inside. After that young mother walks by with her sad little bundle, Rick shares the current theory that Patrick got sick, died and attacked Block D. He hurriedly tells Carl he's sorry and they lost a lot of good people. He also hurries to assure Maggie that Glenn and Hershel are fine, but she should stay away from them for a while. As should they all. Rick doesn't offer Maggie an update on her sister Beth, but Maggie probably either figures Beth was safe in C block or doesn't care.