Hershel Greene, played by Scott Wilson, left, and Rick Grimes, played by Andrew Lincoln, star in “The Walking Dead.” (Gene Page/AMC)

Note: This recap contains spoilers.

Not feeling well? It’s flu season for the prison-dwelling survivors in “The Walking Dead,” too.

This week, we find out just how dangerous the super-flu is that turned Patrick into a zombie. The bug is suddenly running amok in the prison, and at this point, everyone is at risk of exposure. Sasha falls ill, Glenn falls ill, and even little Lizzie is sick and sent into quarantine.

The desperate situation calls for antibiotics, and Hershel knows a local veterinary college 50 miles away has what they need. Daryl leads the charge on the trip, recruiting Michonne and Bob to join him. Even Tyreese, who desperately wants to bring Karen’s killer to justice and has a sick sister to worry about, decides to join.

The group fails to make it to their destination after being confronted by a mob of walkers on the road. The group tries to reverse the car but gets stuck atop a pile of walkers. (Any fans of exploding zombie heads should have enjoyed that scene). Fleeing the car, Tyreese is seemingly overrun and left behind, but eventually emerges through the woods behind Daryl, Michonne and Bob.

Rick chooses to stay behind from the road trip to keep an eye on Carl and take the lead on finding Karen’s assailant. He discovers that Carol is the killer after comparing the size of his handprint with that of a bloody handprint at the scene of the crime. Rick asks her point blank if she burned Karen and David, another prison member, alive, and she calmly admits to it. We’ve come to love and respect Carol for her unwavering loyalty to the group when it comes to teaching the children and helping run operations and meals. But that loyalty apparently runs deep enough to lead her to think she could protect the group by killing Karen and David.

Hershel also reveals his unfaltering allegiance in this episode. He knows he may have already been exposed to the flu and decides to try and help the ailing survivors at the risk of his own health. After venturing out to the woods to collect elderberry, sporting his trademark sheriff’s hat with Carol in tow, Hershel enters the quarantined area to make tea for the sick. Despite pleading from Maggie, he exposes himself to the flu.

“Listen,” Hershel says to Maggie. “You step outside, you risk your life. You take a sip of water, you risk your life. Every moment now, you risk your life. You don’t have a choice. You can only choose what you’re risking it for. I can help them feel better. I can save lives. That’s reason enough to risk mine and you know that.”

Romance report: Not much but more sad developments here. Maggie is clearly distraught over Glenn and then watches her father sacrifice his health to help the group.

Unanswered questions: Will our group out on the hunt for antibiotics make it back to the camp alive? It’s been a long time since we’ve seen such a large mass of walkers attack. And who was the voice they heard over the airwaves on the car radio on the way to the veterinary college? Finally, what will Rick do now that he knows Carol killed Karen and David?