A silicon-based virus was found on an unnamed planet. It was not really a virus, though that was the closest known classification in Human and Denobulan medical science. Nothing resembling it could be found in the Denobulan medical database.
Since silicon-based life doesn't generally evolve on class M planets, it was theorized that the infectious microorganism was brought there on a meteorite, a theory substantiated by the presence of a meteor crater close to the point of infection. The Organians claimed they had nothing to do with placing the virus on the planet.
The virus was hard to detect, as only thirty-seven percent of the species that contracted the virus in the ten thousand years preceding 2154 identified the pathogen. Amongst its symptoms were passing out, coughing, vomiting, physical pain, sweating, and apparently mental confusion. Due to the high rate at which the pathogen reproduced, death came within hours. The silicon matrix of the pathogen could be disrupted by a specific frequency of radiation, but the dosage would be lethal for Humans. Only seven species discovered this in the ten-thousand-year period, and none in sufficient time to save themselves.
The reaction of visiting crews to the virus had been studied by Organian observers for ten thousand years, though this fact had gone unnoticed by ninety-eight percent of the species. The rationality displayed by a species while dealing with the virus was evaluated and treated as a litmus test to find out if first contact should be made.
The virus had infected several species in the last ten thousand years, including Cardassian and Klingon crews, although the latter was killed by their commanding officer. In cases where the crew did not abandon the infected crewmen, everyone usually died. In 2154, the virus infected Hoshi Sato and Commander "Trip" Tucker from the NX-class starship Enterprise. Ultimately, a cure was found, but not in time, and Sato therefore died. However, impressed by compassion displayed by Captain Jonathan Archer, one observing Organian cured and reanimated Tucker and Sato. Enterprise left a warning beacon in orbit of the planet, in order to prevent further infections. (ENT: "Observer Effect")