Walter finishes talking and stares out into space. "And ... you don't think you belong in a mental institution?" asks Aaron. Jesus Christ, no wonder your mom never comes home and the neighbor won't talk to you. Walter looks like he's trying to smile but he's still sad, but that's nothing that another slug of milkshake can't cure.
Back in the tunnel, the UV squad is setting up the lights for what is actually a very tiny infestation of the fungus. "Doesn't look like much in here. No wonder we overlooked it," says Lee. Olivia continues to look, and spies chalk drawings on the walls, geometric shapes and patterns very similar to what Aaron was doodling. Anyway, everyone steps back so the floodlights can be turned on, and -- really, no contaminant suits? Regardless, the lights are on, and back in the Harvard lab, Aaron starts squinting. "It's bright," he moans. He has to cover his eyes. Meanwhile, Lee is looking through cracks in the walls and asks for a handheld floodlight because there's lot in there. An agent gets one for him, not noticing the fungus slithering up the leg of a tripod holding a floodlight.
Lee shines the light through the cracks, and finds a lot more fungus, while Aaron starts freaking out back at the lab, screaming at Walter to "get away!" as Walter tries to help him.
Lee notes that the fungus is growing down through the grating, and then the lamps start to shatter from the tendrils snaking across the lenses. "Burn it. Burn it all," says Olivia.
Back at the lab, Aaron's on the floor, and Astrid has come over to help. Aaron's got a high-grade fever. A worried Walter tells Astrid that he was complaining about the brightness and now is burning up -- and then it twigs, and he rushes for the phone to call Olivia and tell her to stop the fungicide: "You're killing the boy!" he tells her.
Later, Walter and Astrid have Aaron in an ice bath to bring his fever down (which puts Walter in mind of a popsicle, and a weary Astrid promises to get him one later). Olivia calls for an update and asks if he's positive Aaron was affected by the fungus in the tunnel. He's positive there's a connection, since the collapse and fever coincided exactly with them trying to eradicate the fungus.
So why didn't the boy feel anything when they destroyed the fungus in the hospital? Walter doesn't know yet. "OK, whatever it is, it's dangerous, and we need to destroy it. So how do we break the connection between it and the boy?" Again, Walter doesn't know, but thinks the best way to figure it out is to learn how the connection was formed in the first place. Maybe the fungus is Aaron's brother from another mother? Nah, that seems unlikely, even on Fringe. Olivia says she's got an idea about that.