Rigellian Kassaba fever was a disease. It may be the same disease as Rigelian fever, or it may be a fictional illness altogether.
It was used as a diagnosis by Doctor Leonard McCoy to explain the feigned symptoms of Spock in order to help gain control of the USS Enterprise from the invading Kelvans. According to McCoy, patients who suffered from this disease could experience occasional re-occurrences as many as ten years after their initial infection, but it was treatable provided one received the proper dosage of stokaline in time. (TOS: "By Any Other Name")