The Aphasia virus
The aphasia virus was invented in 2351 by Dekon Elig, a Bajoran scientist, who stored the virus aboard Deep Space 9 as a booby trap intended for its Cardassian occupiers. The virus did not technically induce aphasia, usually caused by physical damage to an area of the brain, but only mimicked its effects in a similar manner. Its symptoms included speaking in gibberish and the inability to understand language. The virus disrupts the brains's processing of aural and visual stimuli. It works by imposing itself within the established synaptic pathways of the temporal lobes and then reroutes them.
Elig had designed the virus, along with his assistant Surmak Ren, to attack the Cardassians and eventually spread to the entire population of the station. It was hoped that inability to communicate or properly read the station's information displays would lead to the destruction of the station.
A result of the Aphasia virus.
In 2369, however, after nearly two decades of lying in wait aboard the station, the virus was inadvertently activated by Miles O'Brien and began to spread aboard the now-Bajoran station. The crisis that Elig had hoped to induce among Cardassians threatened to destroy the station even as it had recently become a place of vital strategic importance to Bajor and the Federation. Jaheel, a freighter captain docked at the station at the time of the crisis, nearly crippled the facility after he became infected and attempted leave the station even though his ship was still attached to the station by its mooring clamps.
In order to stop the virus, Doctor Julian Bashir took several samples and exposed them to different kinds of radiation, including delta and hyperonic radiation. He grew the samples in triagar solution or tetracitrus gel and kept them at 370 kelvins and at an atmosphere of a class M equivalent.
The spread of the virus was only halted through the efforts of Quark, Odo (who were both immune due their physiology), and Kira Nerys who forcibly brought Surmak aboard the station in order to infect him and provide incentive for him to develop an antidote. (DS9: "Babel")