A serious illness that Doctor McCoy pretended Spock had contracted, as a pretext for treating him with stokaline, during the Kelvan seizure of the Enterprise in By Any Other Name.
Rigelian Kassaba fever is a disease said to recur in those who have once caught it, flaring up without warning years afterwards. Whether it is as dangerous as McCoy claims is never tested, since the only case seen on screen is a fabrication.
In the original series episode By Any Other Name, Kelvans from the Andromeda galaxy take control of the Enterprise. Spock puts himself into a deep trance so that he can be carried back to the ship, and McCoy tells the Kelvan Hanar that the Vulcan is having a relapse of Rigelian Kassaba fever picked up a decade earlier and will die without treatment.
The treatment he injects is stokaline, in reality a harmless vitamin compound. The deception gets Spock and McCoy back aboard with freedom to work, and later lets McCoy dose Hanar with stimulants as part of the plan to overwhelm the Kelvans with unfamiliar human sensations.