Sakuro's Disease

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 instability, and eventually coma leading to death. Sakuro's Disease is always fatal if untreated, but as of the 23rd century, it could be cured at a hospital. 

The disease continued to be a menace well into the 24th century, as strains mutated, sometimes rendering even the best medical treatment ineffective. Several cases were documented on Aldebaran Prime from the 2340s into the 2370s. Ann Mallory fell victim to Sakuro's in 2342, orphaning her son, Drake. Caera Shean also died from the disease around 2370. (Kal-Dixas Spaceport, Star Trek: A Call to Duty)