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. More than just a simple credo, for those of the planet Vulcan, it is the cornerstone of their philosophy.
Now, on the Vulcan Science Colony Nisus, that credo of tolerance is being put to its sternest test. For there, on a planet where Vulcan, human, Klingon, and countless other races live and work side by side, a deadly plague has sprung up, a plague whose origins are somehow rooted in the concept of I.D.I.C. itself, a plague that threatens to tear down that centuries-old maxim and replace it with an even older concept.