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 could be found in the Denobulan medical database.
Since silicon-based life doesn't generally evolve on class M planets, it was theorized that it 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 that they had nothing to do with placing the virus on the planet.
The virus was hard to detect, as only 37% of the species that contracted the virus in the last ten thousand years 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. It's Silicon matrix could be disrupted by a specific frequency of radiation, but the dosage would be lethal for humans. Only seven species had discovered this in the last ten thousand years, and none in time.
The reaction of visiting crews to the virus had been studies by Organian observers for ten thousand years, though this fact had gone unnoticed by 98% 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 died. However, impressed by the compassion displayed by captain Archer, one observing Organian cured and reanimated Tucker and Sato. Enterprise left a Warning beacon in orbit, in order to prevent further infections. (ENT: "Observer Effect")