The aphasia virus was a synthetic virus that mimicked the effects of aphasia. The virus did not technically induce aphasia, which is usually caused by physical damage to an area of the brain. The virus resided in the temporal lobes of infected individuals, and had an adaptive synaptic inhibitor. Its symptoms included speaking in gibberish and the inability to understand language. The virus disrupted the brain's processing of aural and visual stimuli. It worked by imposing itself within the established synaptic pathways of the temporal lobes and then rerouting them. In the later stages the virus caused high fever and attacked the autonomic nervous system, which eventually led to death.
The virus was invented by Dekon Elig, a Bajoran scientist, in 2351. The virus was stored on Terok Nor as a booby-trap intended for the station's Cardassian occupiers. He 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.
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 then-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 to 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 Kelvin and at an atmosphere of a class M equivalent. According to Doctor Bashir, the virus varied its gestation period with each new individual. Bashir tried to use base pair destabilizers and sequential disruption against the virus, but none were effective in combating it.
The spread of the virus was only halted through the efforts of Quark and 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")