The IDIC Epidemic is a Star Trek: The Original Series novel written by Jean Lorrah. This novel is especially beloved by Star Trek fans because it provided a very gratifying explanation of why the Klingons seen in the original series have a very different appearance from the "Imperial" Klingons with huge forehead ridges seen later from the movies forward. However, The IDIC Epidemic's explanation is incompatible with the definitive explanations delivered much later during the 4th season of Enterprise, episodes "Affliction" & "Divergence". For about 17 years, this novel's explanation for the discontinuity was the most satisfactory answer fans had, so it remains a notable landmark to those who read it.
I.D.I.C. – Infinite Diversity in Infinite Combination. This is a philosophical cornerstone of the Vulcan society. Currently, a mysterious plague has hit the planet, a plague somehow tied into I.D.I.C and the fact hundreds of races also currently make their home on Vulcan as well. Unfortunately Vulcan is also a central part of Starfleet and the Federation as well, so if it falls, war might follow.