Xenopolycythemia was a rare blood disease, associated with unusual hemoglobin and white corpuscle counts. It caused excessive proliferation of red blood cells in the Human body. The disease was always fatal within a year if not treated early. The cure, possessed by the Fabrini in the 23rd century, was extremely uncomfortable, and the drug had to be administered in small doses at specific intervals during the treatment period. When given in time, recovery was 100%.
In 2152, when Captain Archer was kidnapped and put on trial by the Klingon Empire, Doctor Phlox told them that Archer had xenopolycythemia, a ruse that allowed him to meet with the captain. To convince the guard to leave, Phlox described the disease as highly contagious, and said that they should have put Archer in quarantine. (ENT: "Judgment")
In 2268, Dr. McCoy was diagnosed with xenopolycythemia. Believing himself to not have long to live, he was susceptible to the chance for happiness which marriage to Natira, the Fabrini high priestess, afforded him. After Spock discovered a cure among the technological lore of the Fabrini, McCoy separated from his wife and returned to the USS Enterprise. Nurse Chapel administered a hypospray with the new medication, and reported that his white corpuscle count was back to normal. Spock, after consulting his tricorder, added that "your hemoglobin count is back to normal, doctor, which indicates that the flow of oxygen to each cell of your body is back up to its abundantly energetic level." (TOS: "For the World is Hollow and I Have Touched the Sky").