The life prolongation complex was an ambitious attempt at bioengineering that went terribly wrong.
The life prolongation complex was a series of viral infections that, in a chain reaction, would greatly extend the lifespan of an ordinary cell. The goal of the complex was to extend the life of the cell so that an individual would age approximately one month each hundred years. Depending on exactly when the complex was introduced, a Human could expect to live around 80,000 years, barring accidents.
The Onlies, feral children still alive on Miri's world in 2266, demonstrated that the complex did prolong the lives of children; these individuals had been born about three hundred years earlier, and had not aged much in the interim. But the complex had a fatal flaw: it killed adults.
The fatal form
The complex killed all the adults on Miri's world – they developed a rapidly progressing, extremely debilitating disorder of the metabolism from which no anatomical system was spared. Children developed this form when they began entering puberty. While Humans are vulnerable to this disease, Vulcans are not, although they can be carriers.
The first symptoms of the complex are cosmetic: inflamed bluish blemishes with red margins appeared. In some individuals, these blemishes appeared within hours of exposure. Within a few days, internal symptoms developed: intense fever, severe pain in the extremities, and blurred vision.
Emotional instability and extreme irritability were the hallmarks of the middle stage. Early symptoms grew more pronounced, with higher fevers and larger, darker blemishes. These symptoms appeared within two or three days of onset; those older when infected experienced a more rapid progression.
In the end stages, about seven days after onset, blemishes covered the entire skin surface, the metabolism ran very rapidly and the sufferer became completely insane. Individuals in this stage attacked anyone they found, and destroyed their surroundings. The metabolic dysfunction left them weakened; in this stage, even a phaser set to stun could be fatal. The resulting "grups" were barely recognizable as human. The Onlies saw all grown-ups become these creatures, consequently, they developed a profound fear of and deep distrust for adults.
In 2266, Doctor McCoy of the USS Enterprise developed a vaccine that cured the fatal form of the disease. At the time, he was out of communication with his ship, and so elected to try the vaccine on himself, without testing it. Fortunately, it worked as expected, although not without side effects of pain and unconsciousness. (TOS: "Miri")