Mirror McCoy died from xenopolycythemia

In the Mirror Universe, Dr. Leonard McCoy died in 2269 of the fatal disease known as xenopolycythemia. This indicated that, in the Mirror Universe, the ISS Enterprise never encountered Yonada, or they simply destroyed the vessel because the Yonada was hostile. McCoy was succeeded as the ISS Enterprise's chief medical officer by Dr. Jabilo M'Benga, who continued to serve in that position until at least 2287, when Captain Saavik was in command of the ISS Enterprise.