Xenopolycythemia is an extremely rare disease in which the spleen enlarges and both red and white blood cells are overproduced. If the spleen is removed within three months, the prognosis is good, but if not treated, the life expectancy of a person with xenopolycythemia is one year. (TOS - Section 31 novel: Cloak)
After Captain Jonathan Archer was kidnapped and put on trial by the Klingons in 2153, Doctor Phlox told a Klingon guard that Archer had xenopolycythemia and that the disease was contagious as a way of seeing Archer. (ENT episode: "Judgment")
On Stardate 2231.2, Doctor Karen Patterson published the article "Searching for Answers: Hope for Humans with Blood-Based Hyperplasia" in Starfleet medical journal issue 421. This article dealt with xenopolycythemia along with several other diseases.
In 2268, Doctor Leonard McCoy discovered that he had xenopolycythemia after performing a routine physical on himself. He encountered Karen Patterson when the Enterprise visited a secret cloaked science station in the Lantaru sector. Patterson had been thinking about the problem on and off for years, but there was no known cure. She was willing to leave the station and work on the problem while McCoy was ill, but she was killed a short time later when the station exploded after attempting to control Omega molecules. (TOS - Section 31 novel: Cloak)
Shortly after this, a cure for the disease was discovered on the Fabrini generational ship, Yonada, by Commander Spock of the USS Enterprise. Doctor McCoy was the first person to be cured of the disease. (TOS episode: "For the World is Hollow and I Have Touched the Sky")
In the mirror universe, Doctor Leonard McCoy died due to the xenopolycythemia in 2269 as, in that universe, the cure for the disease had not been discovered. Jabilo M'Benga succeeded him as the chief medical officer of the ISS Enterprise, continuing to serve in that position until at least 2287. (TOS - Mirror Universe novel: The Sorrows of Empire)