The Vulcan time of mating, a biological drive that seizes adult Vulcans roughly every seven years, strips away their logic and can kill them if it is not resolved.
Pon farr is the Vulcan time of mating. About once every seven years an adult Vulcan is overtaken by a neurochemical imbalance that erodes emotional control and ends in the plak tow, or blood fever. Unless the drive is resolved by mating, by ritual combat or by intense meditation, it can be fatal. Vulcans treat the subject as deeply private and rarely discuss it with outsiders.
The condition is introduced in Amok Time, when Spock must be rushed home to Vulcan in 2267. The rapidly ageing young Spock suffers it on the Genesis planet in Star Trek III: The Search for Spock.
Later stories return to it: Vorik and B'Elanna Torres in Blood Fever, Tuvok in Body and Soul, and T'Pol, in whom a microbe triggers it early, in the Enterprise episode Bounty.