Lieutenant Bird and Commander Matheson had been exposed separately on Earth; coincidence and stray radiation had allowed the viruses to recombine into something new. The mutated virus spread rapidly on the space station; no one was immune. The med bot analyzed the outbreak, who had first shown symptoms, how quickly it passed from person to person.
PLG-01 cross-checked the antibody titers of the crew, but the disease had yet to run its course. The bot sealed access to the docking ports for the shuttles. It could not allow anyone to leave the station for help and risk infecting the Earth.
Author’s Note: I’ve been playing with the idea of Four Robots of the Apocalypse for a while, and I finally decided how to make them work. Robots and computer subroutines wouldn’t be malicious, just practical and literal. That’s pretty scary.