"'Bubonic' implies something very specific. You cannot just use it for a synonym for 'diseased'-"
Ku'Gath is the single most emo daemonic entity in a universe full of immortals with daddy issues. Once, he was but a Nurgling s(h)itting on Nurgle’s shoulder. Whilst Nurgle was mixing his most virulent disease ever, Ku'Gath promptly did a barrel roll, and tumbled from his place on Papa Nurgle's shoulder into the cauldron, where he took a great draught of the lovely contents and swelled with its power. Invigorated, Ku'Gath drank and drank until the cauldron was empty (the fat shit) and he had grown into a Great Unclean One. Nurgle laughed at the antics of his Nurgling, who had become the embodiment of the perfect disease that had been bubbling within the cauldron. Ku'Gath, however, realised that he had robbed his father of the greatest disease ever concocted (essentially making him emo-Obelix), and has since been wandering the universe, searching for the perfect ingredients to recreate the toxin for Nurgle. He also captures the guys he infects and takes them back to his lair, where he can observe their degeneration more closely.
He met Mortarion when the Primarch came upon him crying, in despair, in the Garden of Nurgle. Upon asking why he was sad, Ku'Gath told him about how he became a Great Unclean One. After they talked for a bit, Ku'Gath told Mortarion where he could find his foster father's soul. They bonded over shared daddy issues and even Mortarion, who hates warpcraft, thinks that Ku'Gath is a pretty cool guy and--in words attributed to him--"relateable."
At some point, Ku'Gath was summoned on the Agri-World of Curwen at the head of the Daemonic Legion. A Grey Knights strike force arrived, and Anval Thawn led a force into the very heart of the Daemonic horde to prevent being surrounded. Ku'Gath swallowed the dude whole, but Thawn detonated his weapon inside the Daemon's throat and burst from its body, and unsurprisingly, Ku'Gath was slain and banished back to the Warp. Ku’Gath appears again in M41, when intervention of Styer and his Grey Knights in the attempts of Inquisitor Malia Orbiana to find biological weapons against Orks (which long story short, the Orks were immune to), which led to the manifestation of the Bonewrack disease, Ku’Gath’s once lost infection, in realspace. The Mary Sues managed to defeat Ku’Gath and his horde of Plaguebearers and expel daemons to the warp, while Ku’Gath himself was wrecked by the blow of Librarian Gared when his Nemesis Greatsword bisected Ku'Gath's head. He later led Nurgle's forces in the Invasion of Ultramar during the Plague Wars just to get wrecked again by the even bigger Mary Sues. In fact they banished his ass so hard back to the immaterium that he lost his priviledged status as Papa Nurgle's favourite son and instead has to hang about for a few millenia to be reincarnated.