I'll dive straight into the plot.
Tyreese is horrified to find Karen and David's burnt corpses outside, and tells Carol, Daryl, and Rick that he wants the murderer found. He is so upset, in fact, that he gets into an altercation with Rick over it. Rick goes a little crazy, beating up on Tyreese until Daryl can pull him off.
Meanwhile, the infection that killed Patrick is still spreading. Although initially hopeful that the deaths of Karen and David may have cut the infection off at its source, the group soon discovers the extent of their dilemma. When Sasha starts to show symptoms, she goes to find Dr. S and the other patients. She discovers that everyone, including the doctor, is violently ill. All of the patients remain in isolation, and the children and the old (aka Hershel) are put in isolation elsewhere, in an attempt to stop the sickness from spreading. Glenn begins showing symptoms as well, to Maggie's horror, as does the little girl, Lizzie, whose father died in the previous episode.
Hershel states that the disease itself isn't what's killing anyone, but rather the symptoms. If they can get antibiotics, they might have a chance. Daryl, along with Michonne, Bob, and Tyreese, who is still angry about Karen's death, decide to go try and find medicine. On their way to a veterinary school, they are unexpectedly surrounded by an enormous horde of Walkers. They have to abandon their car, but they all manage to escape, at least for the time being.