The Klingon augment virus was a modified form of Levodian flu that threatened to wipe out the Klingon race in the mid-22nd century. It was inadvertently created by Klingon researchers who were attempting to bio-engineer enhanced warriors using DNA from genetically-modified human embryos left over from Earth's Eugenics Wars.
In 2154, Klingon scientists working under the direction of Antaak recovered several Augment embryos from a hijacked Bird-of-Prey destroyed in orbit of Qu'Vat colony during the Augment Crisis. Fearing that Humans could overwhelm the Empire if they moved to staffing their ships with Augments, a project was set up to attempt to create their own Augments to counter the perceived threat.
The effort failed dreadfully. The Augments created did develop enhanced strength and intelligence, but they also began to show Human characteristics, from the blatant to the subtle. These included personality alterations and loss of the characteristic forehead ridges. Ultimately, the subjects died agonizing deaths when the incompatible DNA resulted in neural system breakdown. While working to stabilize the process of augmentation, the scientific team augmented a test subject who was, unknown to the team, also infected with Levodian flu. The flu virus in some manner incorporated the augment DNA, turning a carefully controlled experiment into an epidemic. As the unstable augmentation process always killed the subject, this epidemic threatened the Klingon race with extinction.
The virus spread very rapidly, infecting millions within months. One of the first planets struck was N'Vak colony. The Klingons attempted to capture Doctor Arik Soong to help develop a cure, but their efforts were thwarted by Soong's incarceration in a high security detention. They then moved to kidnap Doctor Phlox, using Rigelian intermediaries and the assistance of Section 31 agents. Phlox was forced to work with Antaak to find a cure. However, the Klingon High Council was unwilling to wait, and dispatched a fleet to annihilate the infected planets, starting with N'Vak colony. Afterwards it set course to Qu'Vat colony. (ENT: "Affliction")
In 2154, Klingon scientists working under the direction of Antaak recovered several Augment embryos from a hijacked Bird-of-Prey destroyed in orbit of Qu'Vat colony during the Augment Crisis. Fearing that Humans could overwhelm the Empire if they moved to staffing their ships with Augments, a project was set up to attempt to create their own Augments to counter the perceived threat.
The effort failed dreadfully. The Augments created did develop enhanced strength and intelligence, but they also began to show Human characteristics, from the blatant to the subtle. These included personality alterations and loss of the characteristic forehead ridges. Ultimately, the subjects died agonizing deaths when the incompatible DNA resulted in neural system breakdown. While working to stabilize the process of augmentation, the scientific team augmented a test subject who was, unknown to the team, also infected with Levodian flu. The flu virus in some manner incorporated the augment DNA, turning a carefully controlled experiment into an epidemic. As the unstable augmentation process always killed the subject, this epidemic threatened the Klingon race with extinction.
The virus spread very rapidly, infecting millions within months. One of the first planets struck was N'Vak colony. The Klingons attempted to capture Doctor Arik Soong to help develop a cure, but their efforts were thwarted by Soong's incarceration in a high security detention. They then moved to kidnap Doctor Phlox, using Rigelian intermediaries and the assistance of Section 31 agents. Phlox was forced to work with Antaak to find a cure. However, the Klingon High Council was unwilling to wait, and dispatched a fleet to annihilate the infected planets, starting with N'Vak colony. Afterwards it set course to Qu'Vat colony. (ENT: "Affliction")
When it became clear to the research team on Qu'vat colony that it would take weeks to develop a cure, stabilizing the augment virus became the researcher's primary concern. Their efforts proved successful; a method was developed that stabilized the process in the early stages – after the cranial ridges had dissolved and some minor neural re-ordering had occurred (which caused personality changes), but before the virus became contagious, enhanced augment abilities and fatal neural pathway breakdown formed.
Eventually, Admiral Krell convinced the High Council to call off their sterilization program of destroying the infected planets. The cure was distributed throughout the Empire. Because afflicted Klingons' DNA had been altered by the virus, the Human appearance and genetic traits were passed onto the descendants of the infected. Initially there were millions of Klingons who had to live with the disfigurement. (ENT: "Divergence")
A group of Klingons descended from those infected with the augment virus in 2267
In 2154, Antaak and Laneth feared that Klingons without ridges would become outcasts, lose their positions and would no longer have a place in the Empire. (ENT: "Divergence") However, by the 2260s, affected Klingons served as the captains and crews of front-line starships in the Klingon Defense Force. (Star Trek: The Original Series) Later on, some of them even achieved the title of Dahar Master and earned their statues among the Hall of Heroes on Qo'noS. (DS9: "Blood Oath", "The Sword of Kahless")
By the 24th century, knowledge of Klingons with smooth foreheads had been generally forgotten by the population of the United Federation of Planets. Also, the reason for Klingons having smooth foreheads a hundred years earlier was not widely known outside the Empire. When asked by Miles O'Brien and Julian Bashir, Worf simply gave the answer "we don't discuss it with outsiders", though Bashir immediately suspected viral mutation to be the cause. (DS9: "Trials and Tribble-ations")