Dekon Elig was a Bajoran geneticist and resistance member who engineered the aphasia virus and hid it in the replicators of Terok Nor, where it lay dormant until Starfleet's arrival.
Dekon Elig is the unseen author of the crisis in the first-season episode Babel. A scientist working with the Bajoran underground during the Occupation, he designed a virus that scrambles its victims' speech and understanding before killing them, and concealed a device to release it in the station's food replicators while Terok Nor was being built, some eighteen years earlier.
The device was never triggered until Chief O'Brien's repairs set it off, infecting Starfleet and Bajoran personnel instead of the Cardassians it was meant for.
Kira's search for an antidote reveals that Dekon died years before while trying to escape from a Cardassian prison camp. The trail leads instead to Surmak Ren, his former assistant, whom Kira kidnaps and infects in order to force his help.