Scully tells Dr TaeBo to get the CDC on the phone, so that they can learn what "sets this thing off." TaeBo hops to. Next, Scully rips into Karas, who's just entered the room, telling him to blockade any and all exits from the line, citing a "possible outbreak situation." Karas grouses that it's not going to happen. Scully yells that it is. Karas tells her it isn't, because he already has passengers on the platform, waiting for the train. Scully turns to look at the monitors of the stops, which do indeed show people streaming into the station. She makes a frustrated face and kicks Karas in the shins. Okay, she just makes the face. "Agent Doggett," she says, "we've got a new wrinkle."
Down in the tunnel, Doggett grimly searches for Dickhead as he and Scully discuss what the catalyst for the outbreak could be. Scully wonders if it's body temperature. Or heart rate. Or too many McRibs the night before. Doggett grimly comments that it could be anything -- a grim truth with which Scully agrees. He tells her to hold the trains until he finds Dickhead. She informs him that there are already passengers waiting at the next station. Doggett's in the middle of calling Karas a son of a bitch when he finds Dickhead, passed out in the middle of the tunnel. He's alive, but is slowly being eaten. Scully shortly informs TaeBo that she needs anything -- anything at all -- from the CDC. TaeBo nods; Scully scares her. Scully tells Doggett that she's getting him out of there.
Below, in the hot box, Dickhead wheezily pleads with Doggett not to abandon him. Doggett gives him a dirty look, but helps him up. As they limp down the tunnel, Doggett spies the mysterious "other person" for whom they had been searching, and calls out that whole "federal agent/halt!" rigmarole. Scully watches open-mouthed as Doggett closes in on what turns out to be a child. He and Scully both make amazed faces. The kid doesn't respond to any of Doggett's questions, but just points and gestures. Scully has Doggett turn the light away from the child, so that she and TaeBo can see that the lovable scamp is uninfected. "How can that be?" wonders TaeBo. The proverbial light bulb goes on over Scully's head, as she realizes what the rest of us figured out half an hour ago: sweat triggers the reaction. She spits out some science stuff that boils down to the fact that sweat conducts calcium ions, and that kids aren't as sweaty as grown-up folks. Doggett absorbs this information as the kid gestures for him to follow him into a small room.