FNV-23
FNV-23, known also as the Gray Death, was an Imperial-created virus first tested on the planet Sedesia. Developed under the auspices of the Pathogen-Based Loyalty Enhancement Program by Doctor Fesjo Negleem, this virus had the property of continually changing its antigen needs, making it impossible for an individual to become immune to it as well as making it virtually impossible to cure.
Sources
"The Cure" – Star Wars Adventure Journal 6 (First mentioned)
Worse than Hibernation Sickness 7 Deadly Star Wars Diseases on StarWars.com (backup link)
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