Babel (Star Trek: Deep Space Nine)

Plot

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.

Unfortunately, the doctor (Dekon Elig) who created the virus had died while in a Cardassian prison. One of Dekon's fellow resistance fighters, Surmak Ren - now the administrator at a Bajoran hospital - had witnessed Dekon's death; Kira believes Dr. Surmak may have knowledge of the virus, so she decides to go get him. At first, Sisko refuses to allow Kira to leave (citing the risk of an uncontrolled epidemic), but then Odo recommends to Sisko that she goes because she is "all we have", and may soon be the only person on the station that's still able to communicate. Kira links with a computer in Dr. Surmak's office and transports him to the runabout that she is piloting. Initially, Surmak refuses to help, but relents when Kira reveals that by transporting him to the runabout, Surmak is also infected with the aphasia virus.

Meanwhile, virtually the entire crew is incapacitated. Captain Jaheel, who was desperate to leave to avoid infection, tries to leave DS9 without clearance, and ruptures his power core in the process. To prevent the resulting antimatter explosion from destroying half the station, Odo and Quark eject the ship by explosively releasing the docking latches holding it to the station; the ship explodes at a safe distance, and Surmak is eventually able to create a vaccine for the aphasia virus, based on the initial work of Bashir.