The Klingon Augment virus was a modified form of Levodian flu that later turned into one of the greatest menaces to the Klingon race.
In 2154, a Klingon Bird-of-Prey was destroyed by Human Augments (created by Arik Soong). Klingon military scientists viewed these Augments as a kind of super-soldier being built by the Humans for war, and so they used the Augment DNA to make super-soldiers of their own to counter this threat.
Augmented Klingons gained increased strength (in addition to their twice as strong as Human strength) and intelligence. However, they began to lose their cranial ridges and lost many racial traits (due to neural reordering caused by the Augmenting, the affected behaved dishonorably, acted like "tame" targs and lost the ability to speak/understand the Klingon language). But an even greater threat was when the Augments began dying. Their DNA was then attached to the Levodian flu, which spread throughout the Klingon Empire. Military scientist Antaak ordered the kidnapping of Denobulan Dr. Phlox in order to find a way to stabilize the augmenting process, but Phlox's purpose was to keep the Klingon race from extinction.
In the end, the virus' lethal affects were cured, but not the "neural reordering". Within less than a century, the majority of the Klingon race lost their cranial ridges and experienced major personality changes.
Most notable among the long-term affects was the loss of the distinctive cranial ridges. Klingon hair also did not grow very long (some could not even grow beards, an unnatural thing since all Klingon males grow facial hair), and their teeth were no longer pointed.
One most notable change was the fact that, despite the increased strength among some Klingons, females were actually weakened by the affects. As a result, Klingon rituals with more violent undertones (including par'Mach) were either wholly abandoned or made in mockery of Human rituals.
Affected Klingons also had lower blood-alcohol tolerance levels.
In 2290, it was stated that Klingons lack tear ducts. This is due to the augmentation, which, though it caused Klingons to be more susceptible to "human" emotions such as fear, terror and sadness, robbed them of the ability to express it in any way.
Affected Klingons saw self-preservation as a higher goal than honor, and a restructuring of the Klingon Empire into a fascist state occurred that lasted throughout the earlier half of the 23rd century.
Due to the neural affects, Klingons felt emotions they never knew before, such as fear.
In addition, they could no longer speak the Klingon language, referring to it as "Klingonese" rather than the correct "tlhIngan Hol" which now mainly consisted of more "Human" pronunciations of Klingon words using more K's and Z's then more proper Q,s and S,s and far less strongly enforced syllables.
In 2270, during the fourth year of the famous "Five Year Mission", James T. Kirk, a Human with well-known bigotry against Klingons, was assigned to deliver a cure for the Augment virus to Qo'noS as a peace-offering between the Federation and the Empire. This cure was then mass-produced and the Empire returned to its former glory.
- Simple cranial grafting would not work, since it would not change their strength issues or augmented brain functions.
Klingon augments remained throughout the Klingon Empire until 2346 when K'holl died in the Khitomer Massacre. One such (who was actually the offspring of an augmented father and an unaffected mother) was Chang.