An incurable blood disorder with which Leonard McCoy diagnoses himself in 2268, giving him a year to live until a cure is found in the archives of the Fabrini.
Xenopolycythemia appears in the Original Series episode For the World Is Hollow and I Have Touched the Sky. During routine crew physicals McCoy finds that he has the disease, for which Federation medicine has no treatment, and tells Kirk he has at most a year left.
The diagnosis shapes his choices on the asteroid ship Yonada. He falls in love with its high priestess Natira and decides to stay with her for whatever time remains, accepting the implant that binds him to the Oracle.
Once the Oracle's control is broken, Spock finds the stored knowledge of the Fabrini, which includes a cure for xenopolycythemia. McCoy is treated and returns to duty.