An extremely rare and fatal disease contracted by Assistant Federation Commissioner Nancy Hedford on Epsilon Canaris III, forcing her removal from peace talks for urgent treatment.
Sakuro's disease is the illness at the heart of the Original Series episode Metamorphosis. Nancy Hedford is trying to prevent a war on Epsilon Canaris III when she is found to have it; the chances of infection are said to be billions to one, and she resents both the disease and the interruption.
Kirk, Spock and McCoy are taking her by shuttlecraft to the Enterprise, where she can be cured, when the Companion pulls the Galileo down to a planetoid. Without the ship's facilities her condition worsens rapidly and McCoy can do nothing.
As she lies dying, the Companion merges with her. The joined being is healthy and human, and chooses to remain with Zefram Cochrane.