Pyrrhoneuritis was a very rare disease that affected humans in the 23rd century. Pyrrhoneuritis was first discovered on distant colony worlds, and eventually made its way back to Earth. Prior to a cure being discovered, the disease was always fatal. A person who had pyrrhoneuritis often lost a great deal of weight, and would often be in a great deal of pain as the disease progressed. A cure for pyrrhoneuritis was finally discovered in 2264.
One notable sufferer of pyrrhoneuritis was David Andrew McCoy, father of Leonard McCoy. When first diagnosed with the disease, the elder McCoy was a still robust and wiry 81 year old man. Within three months McCoy had lost about a third of his body weight, was crippled, bedridden, and wracked with pain. McCoy's agony became so unbearable, overwhelming even high doses of painkillers, that he begged his son Leonard to turn off his life support machinery. Leonard McCoy finally complied with his father's wish, but tragically a cure was discovered only a few weeks later. For many years McCoy struggled with the guilt over what he had done. While the younger McCoy shared his pain and guilt with Natira, it was not until 2287 that Sybok was able to help relieve McCoy of this pain and guilt once and for all. (TOS novels: Ex Machina, Star Trek V: The Final Frontier)