Sakuro's Disease

Sakuro's Disease was a very rare, life-threatening illness. According to Leonard McCoy, in 2267, the chances of contracting it were literally billions to one. It was caused by a pathogen of some sort. The disease was detectable long before it was symptomatic. Symptoms included fever, emotional lability, and coma leading to death. The disease was 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")