“Babel” is episode five of season one of Star Trek: Deep Space Nine.
Chief O’Brien is being run off his feet trying to fix everything that’s broken on DS9 – which seems to be everything. When he fixes one of the replicators, a device attached to it activates.
O’Brien starts suffering from aphasia – he loses the ability to communicate intelligibly. When Dax, too, comes down with it, it becomes clear that this isn’t an isolated case, but a virus. More and more people on the station start showing symptoms – not helped by Quark using the affected replicator and selling the food to everyone.
Major Kira finds the device and it is Cardassian in manufacture, but the virus is Bajoran. Presumably, this was an attack by the Bajoran underground that never triggered.
The race is on to find a cure before things get worse – because more and more are being affected, and the earliest victims are going to die.