An overworked Chief O'Brien is attempting to repair numerous malfunctions throughout the station, including most of the station's replicators. The pressure is only escalated when Captain Jaheel approaches O'Brien regarding his overdue repairs and perishable cargo, and Quark badgers him about the replicators in his bar. After reassuring the irate Jaheel (and Quark), O'Brien returns to his repairs, but unknowingly activates a device that was placed in one of the replicators. Shortly afterwards, O'Brien begins showing signs of aphasia — he becomes unable to speak coherently, or comprehend spoken language.

At first, Doctor Bashir is not sure why O'Brien suddenly became aphasic. Tests showed no physiological causes, but when Lt. Dax suddenly becomes aphasic right before his eyes, he soon realizes that they are dealing with a virus that only mimics aphasia. He also determines that the virus is in the food that had come from command level replicators, but that the other replicators aren't producing contaminated food. People all over the station are showing signs of infection as well. The station is put under quarantine to keep the virus from spreading.

Quark now seems to be doing very well—his replicators appear to be working fine. Odo is suspicious, however, and soon discovers that Quark was producing food using a functional replicator in an abandoned set of crew quarters on the command level. The result was that the contaminated food was spread all over the station. As the virus spreads, Bashir discovers that the virus has mutated to become airborne. It also caused a dangerous high fever in its victims, which might kill them.

Kira soon finds a module in the last replicator O'Brien had worked on; taking it to Bashir, he discovers that it wrote the aphasia virus directly into the molecular pattern of any food being replicated, therefore bypassing the biofilers that are used to screen out infectious agents. Bashir reveals that the Bajoran underground placed the module in the replicator 18 years prior, and that the virus was engineered by a Bajoran terrorist during the time of the Cardassian occupation of Bajor.

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Wikipedia