Back in the morgue, David saws through Paul's head, pops out the top of his skull, and removes the brain. Frankly, I'm shocked he still had one. It is dripping with black oily goo. Eee! Alien infection ahoy!
Anyway, now we're back to the scene in the Labitrail lobby where Liam's trying to be all gentlemanly in his own way and Sara's busy stoking his paranoia. This time, Gil appears to be skulking in the shadows behind them. I'm surprised Gil didn't bump into Hodges as the younger man conducted his Mia Vigil.
We're in a conference room where David's explaining, "The black pus suggested a possible infection from an airborne agent. I took the precaution to get you out until hazmat cleared the scene." Sara's all, "Yeah, thanks. I'm going to be having nightmares about the last few seasons of X-Files for a while now." Anyway, as David prepares to tell us all about mucormycosis, we see Hodges. Wow, he and Gil must have just missed each other on the skulking circuit. Hodges hangs out and eavesdrops as David explains, "It's a disease caused by exposure to rhizopus oryzae." This apparently rings a bell with Hodges, who's all, "Did you say rhizopus oryzae?" Liam frets that he and Sara spent hours in the fungi-filled house, and David assures him they're not at risk because their immune systems are healthy. Paul, on the other hand, had a craptacular immune system because he was filled with more steroids than an NFL locker room. For some reason, this news shocks Liam: "Steroids! That guy was stacking to get bigger." You're fired, Liam! Anyway, Sara's curious as to how a mold infection makes Paul look as though someone smashed him across the face. Easy: the mold was eating him from the inside out, turning his bones into soup as it festered in his sinuses. I bet you everyone reading this will freak a little during their next bout of sinusitis.
Anyway, after Hodges points out that rhizopus oryzae likes to grow on human tissue, Sara and Liam conclude that there was some human tissue somewhere in Paul's house: find the root of the fungal infection, and find the tissue source. As the two systematically rip up a wall, they notice the presence of blood, courtesy of luminol, and eventually find the source of the fungal infection: a wet, drippy pipe that's simply coated with the stuff. Liam asks, "How did human tissue get inside a wall?" Sara thinks that's a good question. Conveniently enough, she notices a bullet hole right then. Even more conveniently, it's a 9MM bullet hole. So our working hypothesis is: Paul shot someone, and the bullet with its human tissue payload got wedged in the wall.