Last week, the fourth season of AMC's The Walking Dead debuted to a record number of viewers.  Some 16.1 million people caught up with Rick (Andrew Lincoln) and his group of survivors occupying the prison, and were introduced to this season's "Big Bad".  Rather than a mustache-twirling villain like last year's Governor (David Morrissey), this year's challenge is an even bigger threat despite its microscopic size.  If the title of the episode didn't already give it away, the survivors are now facing a disease of unknown origin.  Hit the jump to see why I've got you under my skin.

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In our recap of the fourth season premiere, I remarked at the attention the writers and new shorwunner Scott Gimple gave to the characters' relationships.  While the main cast got their fair share of screentime in the last go-round, Tyreese (Chad L. Coleman) was only glimpsed.  The follow-up episode features a bit more of Tyreese and his love interest Karen (Melissa Ponzio), with their scenes providing the bookends to "Infected".  Since their arc in this episode perfectly encapsulated whatThe Walking Dead is all about, we'll tackle that first.

After a creepy intro that featured a shadowy figure (one guess on just who that might be) feeding rats to the Walkers at the fence, we got to see Tyreese and Karen in a brief romantic interlude in which Tyreese "sings" the portentous lyrics of "I've Got You Under My Skin", made famous by Frank Sinatra.  Ol' Blue Eyes probably never envisioned that song being a direct metaphor for a deadly infection, but such are the times of the post-apocalypse.  Tyreese waxes poetic about how the world has changed and how now there are so few people that anyone can get close to.  It doesn't take a genius to figure out that things are not going to end well for them.  Tyreese attempts to further their relationship throughout the episode even as all Hell breaks loose around them, but his good intentions are all for naught as Karen succumbs to the new disease and Tyreese finds her burned corpse at the end of a bloody dragging trail as a result.  In this new and terrible world, there's little room for romance, even if it's the only thing that will allow them to survive beyond famine, war, death and pestilence.

Speaking of the disease, the writers really didn't waste any time in letting the characters figure it out.  "Infected" felt like one of those episodes of The Walking Dead in which it remembers that it has its roots in the horror genre.  Karen's wandering through the bowels of the prison with a flashlight on the fritz and Reanimated Patrick's (Vincent Martella) shambling quest for flesh were nods to familiar horror tropes.  The disease itself has been a hallmark of zombie films since George Romero's days, but the protagonists usually don't suss out all the details quite so fast.  It's a good thing they have Hershel (Scott Wilson) around, along with newcomer Dr. Subramanian (Sunkrish Bala), whose medical expertise quickly leads them to the conclusion that some sort of flu-like disease is offing the survivors, who then succumb to the reanimation of the Walker virus.  In a rare bit of science that sneaks onto this show, Hershel correctly mentions that human diseases historically start as infections among livestock, such as pigs and chickens, then eventually make the jump across the species barrier.  The council also tries to take steps to quarantine the infected individuals and prevent further spread of the disease, but they're working with fewer resources than the CDC on furlough.

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Enough science, let's stick with the horror.  Tonight's scene of the Walkers ravaging survivors in the D block was truly horrific.  It was panic-inducing, gruesome and truly terrifying, because it felt like any one of our cast of heroes could have become a victim at any moment.  It was a nice moment to give Rick's crew a stage to show off their maturity and resolve; it also gave Carol (Melissa McBride) a chance to teach some tough lessons in survival training to the next generation.

The Walking Dead has been turning in high numbers of Walker kills because hordes of the shambling corpses have been lining up along the prison fence just waiting to be impaled.  That plot point was worked in nicely with the horde providing a threat to the prison's security, while also allowing Sasha (Sonequa Martin-Green) to notice the pile of dead rats (a mystery is afoot!).  Rick's plan of luring the Walkers away with bleeding pigs was a bit hokey, but it allowed for an interesting crossroads of plot points: the pigs, likely infected, had to be disposed of in some manner so why not sacrifice them to the Walkers; it served as another chance for Carl (Chandler Riggs) to show his maturity as he reacts to the loss of pigs that he'd began to see as pets (as seen by him giving them names); and it showed the weight of decisions Rick and others have to make in this future in order to survive, as the pigs were one of the group's main food sources that unfortunately had to be sacrificed.

There were quite a few nice character moments in this episode, as well.  The relationship between Rick and Carl seems to be smoothing over, as evidenced by the duo re-arming themselves and taking their places of authority back in the group once again.  We also gained an interesting insight into Michonne's (Danai Gurira) past as seen in her interactions with baby Judith; I'm assuming she once lost a child, which has been hinted at in past episodes, I believe.  Carol shows just how strong she's become since the first season and takes advantage of a tragedy by using it as an opportunity to pass harsh life lessons on to the kids.  Rick and Daryl (Norman Reedus) even share a brief moment of mutual respect after the chaos in the prison afforded them the opportunity to work together again.  Bob (Lawrence Gilliard Jr.) remains useless.

Rating: A-  Everything you could want in an episode of The Walking Dead, but it's docked a few points for silly things like Rick allowing potentially infected pig blood to spray him in the face after the rest of the group tried to take precautions like quarantines and masks.  I like the direction the season is taking right out of the gates, but time will tell if this conflict stalls out and/or if anyone of real consequence will lose their lives anytime soon.

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Zombie Kills: ~40, probably more since most of the Walker kills now take place at the fence. (One was probably already dead but Daryl put a bolt through her forehead for good measure; another is dealt with off camera by Rick and his knife.)

Human Enemy Kills: None

Casualties: Prison newcomer #1 (via a reanimated Patrick), Sleepwalkin' Charlie (who was just barbecuing yesterday), Ryan Samuels (aka Lizzie and Mika's father), Karen, David from the Decatur Group, and at least 4-5 others

Best Kill: Crawling zombie stomp by Sasha ()

Best Effect: Fence slicing the Walker's face with his eye squirting through.

Odds & Entrails:

Fun fact: Zombies love rats.

Tyreese: "I got you under my skin. I got you deep in the heart of me. So deep in the heart that you are really a part of me. I got you…"

Rick: "I got other plans involving dirt and cucumbers."

Rick: "No bites, no wounds. I think they just died."

Hershel: "All of us in here … we've all been exposed."

Rick to Carl: "You might want to stay back…" ::Carl runs up and hugs him::

Carol: "We can use cell block A."  Glenn: "Death row? Not much of an upgrade."

Carol: "You okay?" Daryl: "Gotta be."

Mika, on Lizzie: "She's messed up, she's not weak."

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Beth: "When you care about people, hurt is part of the package."

Beth: "All these widows and orphans … what do you call someone who lost a child? You'd think someone would have given that a name."

Michonne holding the baby like a sack of flour was funny as hell for a second, until it got real, real sad.

I'm happy to see that Rick has apparently regained his sanity once again and is stepping up to be a leader once more.

Be sure to tells us what you thought of this episode by posting in the comments below!The Walking Dead airs Sunday nights at 9pm ET on AMC.