Neural parasites spreading from world to world seized the people of Deneva, killing Kirk's brother Sam before the crew found that intense light could destroy them.
In 2267 the Enterprise followed a pattern of mass insanity spreading between star systems to the Federation colony on Deneva. The colonists had been taken over by flying, single-celled neural parasites that attached to the nervous system and controlled their hosts through pain. Kirk found his brother Sam dead and Sam's wife Aurelan dying, while their son Peter survived.
Spock was infected but resisted the creatures through Vulcan discipline. McCoy tested intense light on him, which blinded Spock, before learning that ultraviolet light alone would have worked. Satellites flooded Deneva with ultraviolet light, killing the parasites, and Spock's sight returned thanks to an inner eyelid.