Sakuro's Disease

From Memory Alpha, the free Star Trek reference.

Sakuro's Disease is a very rare, life-threatening illness. According to Leonard H. McCoy, in 2267, the chances of contracting it are literally billions to one. It is caused by a pathogen of some sort. The disease is detectable long before it is symptomatic. Symptoms include fever, emotional lability, and coma leading to death. The disease is always fatal if untreated, but as of the 23rd century, it could be cured at a hospital.

In 2267, Captain Kirk of the USS Enterprise was assigned to retrieve the afflicted Assistant Commissioner Nancy Hedford from Epsilon Canaris III and ferry her to a treatment facility. The shuttlecraft carrying her was diverted off course and soft-landed on a small planetoid by an alien entity called the Companion. The Companion merged with Hedford moments before her death, creating a combined entity, and curing the disease. (TOS: "Metamorphosis")

In the non-canon novels The Captain's Daughter and Serpents Among the Ruins, Demora Sulu's mother is mentioned as having died from Sakuro's Disease.