QuchHa', meaning "unhappy ones", was the name given to the Klingons who has been infected with the Qu'Vat virus, and as a result had a more Human-like appearance due to a lacking of cranial ridges.
Within the Klingon military, QuchHa' generally served in units with others of their kind, though it was not unheard of for units to be a mix of both types of Klingon. (VAN novel: Summon the Thunder)
The QuchHa' were looked down upon by their HemQuch counterparts, and were considered to be less honorable. (ST novella: The Unhappy Ones) Many Klingon warriors had their appearances surgically altered to appear in the more traditional form. (TOS novel: Excelsior: Forged in Fire, DS9 episode: "Blood Oath")
Some QuchHa' were proud of what they had achieved as a group. Kahnrah felt that they had gained superior ambition from their augmentation, and that allowed them to rise to power in the High Council and to the lead the Empire's glorious expansion into deep space. Koloth also felt that the augmentation gave QuchHa' superior abilities over the HemQuch, but felt it best to keep such beliefs to himself. (ST - Klingons - Blood Will Tell comic: "Against Their Nature"; TOS novel: Excelsior: Forged in Fire)
In the 2270s decade, relations between the QuchHa' and HemQuch became particularly strained as a cultural schism erupted between the two groups. Increasingly HemQuch began to take up prominent roles, and played a greater part in encounters with the Federation and Romulan Star Empire, usurping the strong QuchHa' presence at contact with the two powers in previous decades. This internal conflict came to a head when a group of extra-dimensional beings attacked and destroyed a secret Klingon Dreadnought-class starship during the V'Ger incident — resulting in the discovery of the weapon by Federation and Romulan forces, and the subsequent loss of any tactical advantage from the superweapon. The vessel had been the responsibility of QuchHa'; with its loss, the HemQuch group took the political opportunity to gain more power in the High Council and force the entire QuchHa' sub-race into less honorable postings at the extremities of the Empire. (TOS comic: "Debt of Honor")
By the year 2270, a cure had been found to restore many QuchHa' back to normal Klingon appearance. This cure was not effective for all QuchHa', and it was not until the early 2290s that a cure was found that restored the standard Klingon physiology of those for whom the earlier cure had been ineffective. Not all members of the High Council had received this cure by 2293, though the political schism between the two groups seemed to have healed somewhat, with both types of Klingon serving side by side in the High Council. (TOS novel: Excelsior: Forged in Fire; ST - Klingons - Blood Will Tell comic: "Issue 5")