Codex: Chaos Space Marines (2002), p29 — Nurgle Plague Marines

Plague Marines are followers of Nurgle who have chosen to be the vessel for all manner of contagion and pestilence in return for immunity from their effects. Bloated and diseased Plague Marines are horrific to behold but can endure tremendous punishment thanks to Grandfather Nurgle's blessings. The Death Guard Legion was the sole source of all the original Plague Marines although since the Heresy there have been many whose will to live was strong enough to make them sacrifice their souls for continued existence.

Plague Marines are Chaos Space Marines with the Mark of Nurgle. Lords and Lieutenants, Chosen, Possessed, Chaos Space Marines and Chaos Havocs can all bear the Mark of Nurgle, see the Book of Nurgle for more details. Chaos Space Marine squads which are given the Mark of Nurgle will become an Elites choice unless the army is led by a model with the same Mark in which case they remain a Troops choice.