A mysterious virus spreads aboard the station and speech becomes garbled.
While fixing the station's failing replicators, O'Brien unknowingly triggers a hidden device and soon loses the ability to speak or understand language. Dax and many others follow, and Bashir finds the cause is a virus encoded into replicated food, which also brings a dangerous fever. Quark's use of an unauthorised replicator helps spread it, and the virus then becomes airborne.
The device turns out to be an old resistance weapon meant for the Cardassians. Its creator is dead, so Kira brings his former comrade Dr. Surmak Ren from Bajor against his will. Surmak eventually helps Bashir develop a cure before the infection kills the crew.