In the Informal Guide, Dr. Tagore posited a great deal about the history and structure of the Klingon Empire and its culture. One of the most controversial concepts contained in the Informal Guide was that of the "Fusion" Klingons. Dr. Tagore believed that the smooth headed, Human-like Klingons were a genetically engineered Klingon/Human hybrids, and further postulated a whole series of Fusion races including Romulan Fusions. These Fusion races were, he stated, designed to compete with the other races, by blending the best physiological features of another race with those of the Klingon genotype.

Unfortunately, the term "informal" truly suited the book and its concepts. Another term which better suited it was "fictional." Dr. Tagore had never been to the Klingon homeworld, which he dubbed "Klinzhai." The most common reference made to the Klingon homeworld by the Klingons was juHyuQ, which simply meant "home-planet" or "the Homeworld." In fact, it is doubtful whether he had ever even met a real Klingon. He had gathered what sketchy information was abailable about the mysterious Klingons, discarded that which didn't fit his theories and constructed the rest out of whole cloth. When greater contact was initiated between Qo'noS (the actual name of the Klingon homeworld) and the Federation, Dr. Tagore's thesis began to unravel. Unfortunately, there were just enough facts scattered throughout the text of the Informal Guide to the Klingon Empire that it to this day retains some measure of credibility and is still cited by some as an authoritative source.

In fact, the earliest Klingons encountered by the races which would make up the United Federation of Planets were the now-familiar ridge-headed variety. Contact between the Klingon Empire and Earth of the late pre-Federation period was, however, strictly limited. Only a very few Humans had ever met a Klingon.

There is a grain of truth to the fusion theory - the smooth-headed Klingons were indeed the result of genetic engineering, but that's as far as it goes. In 2154, an experiment to create a Klingon super-soldier using augmented Human DNA left over from the late-20^th century Eugenics Wars went awry. The Augment DNA became contaminated with a strain of Livonian flu and spread to the general populace. The mutated virus caused physiological and neural transformations in infected subjects, eventually ending in an agonising death.

The Klingon High Council immediately began destroying the entire population of any infected planet, but the virus continued and spread, threatening to infect the entire Klingon Empire. A Denobulan physician named Phlox, working with a small group of Klingon scientists, managed to create a modified viral strain which eliminated most of the negative effects, although the cosmetic effects could not be eliminated. Entire generations of Klingons bore the physical signs of their shame and dishonour. It took over a century for the Empire to create a final cure which restored the collective Klingon visage to its original appearance. The few Klingons who resisted the cure fled to the Imperial Klingon States within the Triangle, where they maintain a strict quarantine from the Empire. The crew of the IKV BortaS, crash-landed in the Dyson Sphere, were isolated and and are now immune to the cure (in fact, the cure is no longer effective). The Empire and UFP are working on a way to restore the Sphere Klingons.

Klingons, naturally enough, do not discuss this with outsiders. In the late 24^th century, few people in the Federation even remember that the Klingons ever looked different than they do today.