Rigellian plague was a disease prevalent on Rigel II.

It was a strain of Kassaba fever, but had a number of different symptoms that made it difficult to identify when it was first discovered afflicting Rigel II. Its victims died in a slow agony. It decimated a quarter of Rigel II's population.

Rigellian plague may be the same illness as the "Rigelian Kassaba fever" mentioned by Leonard McCoy in a false diagnosis in the TOS episode: "By Any Other Name".

Doctor Haras Keolar, a medical officer of the USS Clara Barton, and other physicians visited Rigel II in 2147 SFC (reference stardate 1/4705, c. 2219) to treat the disease, but saw hundreds of people die from it. But thanks to the recently invented medical tricorder, the physicians were able to quickly diagnose the plague as a Kassaba fever strain and save many more lives. They manufactured a blocking serum on the Clara Barton according to specifications given by the tricorder and began shipments of it to Rigel II. The disease was halted, and they estimated it would soon be wiped out. (ST reference: Spaceflight Chronology, FASA RPG module: The Federation)