Xenopolycythemia

Xenopolycythemia is an extremely rare disease in which red blood cell replacement slows down in the human body. If not treated, the life expectancy of a person with Xenopolycythemia is one year.

After Captain Jonathan Archer was kidnapped and put on trial by the Klingons, 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)

Until 2369 the disease was always fatal, however in that year a cure for the disease was discovered on the Fabrini generational ship Yonada by Commander Spock of the USS Enterprise (NCC-1701). Doctor Leonard McCoy - who had recently discovered that he had Xenopolycythemia - was the first person to be cured of the disease.

In the mirror universe Doctor McCoy died due to the Xenopolycythemia as in that universe the cure for the disease had not been discovered. (TOS novel Glass Empires: The Sorrows of Empire).