Mass insanity

From Memory Alpha, the free Star Trek reference.

A pattern of mass insanity was discovered to be consequential to the spread of neural parasites. Archaeologists discovered the first evidence of this in studies of the ancient civilizations of Beta Portolan. Around 2066, Levinius V was attacked, and then Theta Cygni XII. In 2265, Ingraham B succumbed, and in 2267, James T. Kirk and the crew of the USS Enterprise discovered the parasites when they attacked the Deneva colony. The parasites inflicted enormous pain on those they infected, and this eventually broke the host's mind. As the infiltration spread throughout a planetary population, this effect manifested as mass insanity. Kirk deduced the creature's vulnerability and destroyed them at Deneva, ending the spread. (TOS: "Operation -- Annihilate!")