Gellerpox Infected
The Gellerpox Infected are a new Chaos subfaction allied to Nurgle introduced in Warhammer 40,000 8th edition. They are a recently invented creation of the Plague God's, resulting when a failure in the Gellar Field aboard an Imperial spaceship allows for the all-important field generator to be infected with a biomechanical virus that then proceeds to spread to the workers responsible for maintaining it. It psychically compels them to slowly weaken the generator's anti-Warp field until finally it reaches critical mass, whereupon the field generator transforms into a screaming half-machine half-flesh half-daemon womb and the infected turn into diseased cyborg mutants, who proceed to try and slaughter or infect the rest of the crew.
The codex for the Gellerpox Infected specifically refers to a single cult, that of Vulgrar Thrice-Cursed, and has no real effort put into developing them out for use as Your Dudes, which has mostly contributed to the army, such as it is, being lost to obscurity, since it's more a gloried way to pad out a normal Plague Marine force.
Slaanesh actually found a way to bypass Gellar Fields before Nurgle created this "lovely" infection. The Lady of the Voids came out in Fantasy Flight Games' Dark Heresy's Calixis Sector years before Games Workshop released the Gellarpox Infected.
Vulgrar Thrice-Cursed: Also known as "Old Boilerguts", Vulgrar was originally an engine-master called Amstein Vulg, until the Gellerpox merged him and his two first mates into a single triple-headed multi-limbed abomination of flesh twisted around a pestilential furnace core burning with diseased flames. He is presented as the cult's Twisted Lord, a Gellerpox champion of sorts.
The Vox-Shamblers: Three of Amstein Vulg's most elite followers, twisted into diseased cyborg mutants, characterized by their formidable endurance. It's implied these stand in for the "typical" Gellerpox Mutants band.
The Hullbreakers: Three gigantic, borderline-Chaos Spawn Gellerpox Mutants, who represent the "Nightmare Hulk" unit for a theoretical Gellerpox Infected army that actually exists.
Glitchlings: Daemonic spirits of corrupted code, malfunctioning machinery and rust, essentially cyborg Nurglings that embody computer viruses and other mechanical glitches.
Mutoid Vermin: Three different kinds of mutated, Nurgle-touched vermin swarms; Eyestinger Flies, Cursemites, and Sludge-Grubs.