A pharmaceutical company tests a deadly, I wanna say disease, but actually it's a parasite, maybe the parasite spreads the disease too, but the writer's aren't exactly A+ students in biology, so let's say disease, on a group of prisoners. They give reasons in the episode at to why, but the reasons, like everything else in this episode don't make any sense, so I won't pretend to you that they have reasons.
Nothing about this episode is good. It's not worth anyone's time going into, let's just say, Scully is an idiot, everyone in the government is an idiot, there's this shoehorned 'we gotta tell the public about this conspiracy' plot that sucks and is embarrassing for the writers, and a guy with plague pustules runs around a city and nobody even wonders about it.
If you like terrible schlock this is for you. There's no redeeming quality in the storyline or the action, it isn't tense, it isn't thrilling, you don't care about any of the characters involved, but people have boils on them that invariably explode onto other people's faces. And if that gives you some kind of thrill, I won't question you as to what kind of thrill, but if that gives you some kind of thrill, then according to the other reviews of this episode, you'll definitely think it's a 9 or 10 out of 10.