Glenn and Maggie dig graves in the prison yard. They stop and share a sad smile, then continue to dig.
Dr. S, meanwhile, examines an infected victim. He shakes his head and unsheaths his knife.
Tyreese shows Rick, Carol and Daryl the charred bodies of Karen and David. Enraged, he demands that Rick track down the killer. Rick tries to calm Tyreese down, but Tyreese punches him in the face.
Rick instantly lashes back, beating Tyreese to the ground. Rick punches again and again in a blind rage until both his hand - and Tyreese's face - are mangled.
Afterwards, Hershel dresses Rick's hand and alerts him to a council meeting the following morning. "We could be facing an outbreak," Hershel says.
Tyreese furiously digs graves for Karen and David, ignoring Bob's suggestion he have his wounds checked.
Speaking with Hershel, Glenn seeks reassurance that the outbreak can be contained. Hershel, however, is unsure. They run into Sasha, who waves them away as she's seized with a coughing fit.
Sasha stumbles into Cell Block A and finds Dr. S, who's also contracted the illness. "We need to tell them," he says. "It's starting."
At the council meeting, Hershel says it's the symptoms - not the illness itself - that's killing their people. He suggests they seek antibiotics at a vet college 50 miles away. Daryl and Michonne volunteer for the mission. Hershel also suggests they quarantine the group's most vulnerable members, meaning the children. "What about the old?" Glenn asks.
In the prison yard, Carol and Rick work at a water pump to no avail. Carol notes they'll need to go outside the fence to unclog the hose. "We've got enough for now," Rick insists. "That's tomorrow's problem."
Rick goes to Tyresse and reassures him they'll find the killer. "But we have to save lives first," he says. "We have to keep this place going." Tyreese dismisses Rick's argument.
Carl grudgingly packs his bag to join the kids in quarantine. Rick says he needs Carl to watch over the children, and asks him to report back if anyone gets sick. Carl warns that he may need to use his gun.
Glenn grimaces in his cell and waves Maggie away as she walks in. "I have it," he says grimly.
As Michonne helps Daryl prep the car for the antibiotics run, he calls her out for always "running off" on her hunt for the Governor.
Daryl tries to enlist Tyreese for the run, but Tyreese is determined to keep close watch over the infected survivors - lest the murderer strike again.
In the administrative building, Hershel restlessly rifles through desk drawers until he finds a mug that reads "Java Saves." Hatching a plan, he resolves to sneak out to the woods. Carl insists on joining him for protection.
Carol escorts sick survivors to Cell Block A. She comforts Lizzie before sending her in. Then alone, Carol breaks down in tears.
Beth, who's looking after the children, talks to Maggie through a door in the administrative building. Maggie tearfully reports that Glenn is ill. "We don't get to be upset," Beth says. "We all got jobs to do."
Carl guards Hershel as he picks berries in the forest. He spots walkers approaching and readies his gun. "You don't need to," Hershel says as they walk away.
From the viewing gallery of the execution chamber, Tyreese comforts Sasha, who sits ailing on the other side of the glass. Sasha optimistically envisions Daryl and the others returning with antibiotics as early as tomorrow.
Daryl enlists Bob on the run, saying they need someone with medical expertise to gather the correct meds. Tyreese shows up and asks to join the group. "Hell yeah," Daryl agrees.
Before leaving, Tyreese asks Carol to look after Sasha while he's gone. She expresses her condolences for Karen. After Tyreese leaves, Carol knocks over a water tank in frustration and cries.
Maggie and Rick urge Hershel to go back into quarantine, but he insists on helping the sick with the elderberries he picked - they're a natural flu remedy, he explains. Maggie reluctantly lets him enter the sick ward.
Maggie tearfully tells Beth that Hershel left quarantine. "We all got jobs to do," Beth reiterates. "We don't get to get upset." She tears up in spite of her words.
Rick examines the spot where Karen and David's bodies were found. He follows the trail of blood and notices a bloody handprint on the door. The hand that made it is much smaller than his own.
Alone, Carol unclogs the water hose outside the perimeter fence but attracts a group of walkers in the process. Rick rushes to her rescue. "We decided to do that tomorrow," he scolds once they're safely inside the fence.
On the road, Daryl fiddles with the car radio and hears a woman's voice. "Keep alive," it says. Then "sanctuary" and "Terminus." He slams on the brakes as they encounter an impenetrable herd of walkers. They jump out of the car.
Tyreese, however, stays seated, catatonic. Suddenly, he leaps from the car and begins methodically hacking away at the walkers with his hammer. "Come on!" he screams at the undead masses.
Daryl, Michonne and Bob flee into the woods as walkers surround Tyreese. Moments later, they hear a rustling in the bushes. Tyreese emerges - covered in walker guts but alive.
Hershel feeds Dr. S a cup of elderberry tea. Dr. S coughs and sprays blood all over Hershel's face. Later, Hershel gives tea to Glenn and urges him to stay positive.
Rick confronts Carol as she brings buckets of water to the prison. "Did you kill Karen and David?" he asks. "Yes," she answers matter-of-factly. They stare at each other a moment, then Carol continues inside.