The pathogen of Rigelian fever.
Rigelian fever is a disease originating from the Rigel system. The only known cure for this fever is the rare mineral ryetalyn.
In 2152, Crewman Fisher came down with Rigelian fever after a visit to Tessik Prime, causing Phlox to inoculate the entire Enterprise crew before an outbreak occurred. This would eventually save the life of a crew member, though not in the way intended: after Travis Mayweather was kidnapped by an Automated repair station hoping to use his brain functions to enhance its capabilities, throwing the crew off by creating a near-perfect "dead" replica, the complete absence of the vaccine in his bloodstream (the supposed cause of death would've actually strengthened it) proved that the corpse in sickbay was not the real Mayweather, allowing Archer and T'Pol to rescue him before the brain damage incurred by the station was irreversible. (ENT: "Dead Stop")
In 2154, a sample of this disease's pathogen was stored at Cold Station 12 for study. Malik identified this as one of the most dangerous pathogens kept at the station and transferred them aboard his Bird-of-Prey for "a little insurance." (ENT: "Cold Station 12")
In 2269, there was an outbreak aboard the Federation starship USS Enterprise. With her entire crew imperiled, the ship went to the planet Holberg 917G for its natural ryetalyn deposits. These however were laced with irillium, which if not removed negates the antitoxic effectiveness of ryetalyn. The former immortal Flint, master of Holberg 917G, was told of this fever's similarity to Bubonic plague and recalled the horror at Constantinople in 1334. (TOS: "Requiem for Methuselah")
See also
Apocrypha
Rigelian fever (spelled "Rigellian" fever, though) was a plot device in two of the twenty story arcs in the Star Trek newspaper comic strip