An engineered virus planted in the station's replicators by the Bajoran underground during the occupation, which scrambles its victims' speech into nonsense before killing them.
The aphasia virus is the threat in the Deep Space Nine episode Babel. It was created by the Bajoran scientist Dekon Elig and hidden in a device in the command-level replicators while the station was being built, as a weapon against the Cardassians. It was never triggered until Miles O'Brien, overworked and repairing failing systems, sets it off by accident eighteen years later.
Victims first lose the ability to use language, speaking and writing jumbled words, and then develop a fatal fever. The virus mutates to spread through the air and soon most of the station is affected, while Quark's illicit use of a crew replicator carries it to the Promenade.
With Bashir struck down, Kira tracks down Elig's former assistant Surmak Ren, kidnaps him and exposes him to the virus so that he has to produce the antidote.