A virus designed by the Bajorans 18 years earlier to infect the Cardassians is inadvertently released into DS9's food replicators and atmosphere, eventually infecting everybody on board the station. Once Bashir diagnoses it, the episode becomes a race against the clock to find a cure before the incubation period expires and the virus begins killing people.
The "race against the clock" is not a particularly effective part of this story, because we all know DS9 is not about to become a floating morgue. The ending, where an antidote is all-too-easily and quickly created (and then administered between scenes with a cut to the exterior of the station) goes a long way toward destroying any remaining sense of danger.
On the other hand, a lot of the character details within the plot work nicely. Kira's tracking down the Bajoran experts on the virus is plausibly handled and interesting (as is the way she kidnaps the man who may be able to find the cure). Odo and Quark continue to display their camaraderie-in-code. Sisko and Jake are believable as father and son, with scenes that resonate. And Colm Meaney's O'Brien is terrific in the opening acts, faced with a broken-down nightmare of a space station where nothing works right. But what's most interesting is the virus itself, which has an inspired, aphasic side effect that causes a breakdown in verbal communication, reducing everyone to babbling incoherence.