A family of lethal plague strains, samples of which the Enterprise-D carries in 2365 so that a cure can be developed for an outbreak in the Rachelis system.
Plasma plague is the hazardous cargo in the Next Generation episode The Child, which opens the second season. An unclassified strain is spreading through the Rachelis system, and the Enterprise collects specimens of every known variety from 'aucdet IX for the medical trustee Hester Dealt.
The samples include mutated strains bred for biological warfare, and any leak would kill the crew within hours. Geordi La Forge builds a containment module for the purpose.
In transit one specimen begins to grow under the influence of Eichner radiation. The source proves to be Ian Andrew Troi, the alien life-force living as Deanna Troi's son, who lets his human body die so that the ship will be safe.