A swift and lethal disease resembling bubonic plague that infected the Enterprise crew in 2269, curable only with the rare mineral ryetalyn.
Rigelian fever is the epidemic that drives the Original Series episode Requiem for Methuselah. By the time the story opens three crew members are dead and twenty-three more are ill, and McCoy warns that the whole ship could be lost within hours.
The only known antidote is made from ryetalyn, and the nearest source is the small planet Holberg 917G. There Kirk, Spock and McCoy meet the reclusive Flint, whose robot gathers and processes the mineral while its master delays them for reasons of his own.
The first batch is contaminated with irillium and useless, but a pure supply is finally obtained and the outbreak is brought under control.