Xenopolycythemia was a rare blood disease, causing 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 he had xenopolycythemia so that he could meet with him. He said that it was highly contagious, and that they should have put him in quarantine to get the guard to leave. (ENT: "Judgment")
In the 2150s, a sample of this disease's pathogen was stored at Cold Station 12. (ENT: "Cold Station 12")
Dr. McCoy contracted xenopolycythemia in 2268, but Spock discovered a cure among the technological lore of the Fabrini. Chapel administered a hypospray with the new medication, and reported that his "white corpuscle count is 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").