Plasma plague was a lethal disease caused by a group of viruses known since the 2260s. The pioneering research was conducted on them by Dr. Susan Nuress in 2295 following an epidemic on Oby VI. One mutated strain, developed during Nuress's explorations, was found to grow more rapidly when exposed to eichner radiation.
Plasma plague threatened the heavily populated Rachelis system in 2365. A strain similar to what Nuress found threatened the USS Enterprise-D, when the ship was transporting specimens of plasma plague to Science Station Tango Sierra to combat the Rachelis system contagion. (TNG: "The Child")