Martha returns to House's office and tells him that she doesn't think Julie has smallpox, as there are no pustules on her palms or feet. Her stepdad, however, does have pustules there. House doesn't see how Julie could have something that looks like smallpox but isn't while her stepfather has actual smallpox, but then the Minor Epiphany face appears and House takes off. Martha just stands there with her mouth open so House has to come back and tell her to follow him. They find Taub and Foreman, if not Chase, hanging out in the locker room. House says the stepdad has smallpox because they gave it to him, not Julie. If his immune system was compromised when they gave him the smallpox vaccine, it would explain his extreme reaction to it now. And House actually read the guy's file so he knows that he had kidney cancer six years ago and apparently didn't have Lamby to comfort him during the treatment, so the kidney cancer came back with no one knowing, ruined his immune system, and thus gave him the vaccine version of smallpox. House says all they have to do to test this is give stepdad some of that magical interferon and see if he gets better. If he does, House is right and they're back to figuring out what's wrong with Julie.
Of course, Broda isn't about to let House near the stepdad to give him the interferon, and explains his reasoning by telling us all a story of Janet Parker, who, in 1978, was accidentally exposed to the smallpox virus thanks to some carelessness in a nearby research lab and a very efficient vent system that carried the virus to Janet's office. She was the last known person to die from smallpox. The man in charge of the lab where this happened felt so bad about it that he killed himself. Broda doesn't mention this, but Wikipedia tells me that poor Janet was actually immunized against smallpox 12 years before her death and the strain of smallpox she was infected with was one of the most lethal. Also, her mother was the only person who had contact with Janet that contracted smallpox, and she actually survived. So I guess it's not that contagious, really. Anyway, House could not look more bored by Broda's story and decides to focus on the stepdad instead, noticing that he is now peeing blood which House decides must prove that he had kidney cancer. Broda still refuses to open the door for House or apparently let anyone else inject the stepdad with interferon, so House takes matters into his own hands and walks on in with no safety gear on. Broda presses a red alarm button but can do little else but watch as House administers the interferon. House says he knows he's right about this. Broda says he'd better be, because now he's stuck in that room for the foreseeable future.