Mark of Nurgle

The  Mark of Nurgle is a Mark of Chaos given to  Nurgle 's greatest followers causing them to become a living host to the powers of death, disease and decay. Heretics with the Mark of Nurgle are marked with either a physical mark corroded into their corrupted flesh or a hidden but no less permanent mark upon their soul. The Mark of Nurgle constantly releases pus like a festering wound, without taking any attention away from the bearer. A vast portion of  Chaos Space Marines  with the Mark of Nurgle are from the  Death Guard Traitor Legion   and their warbands. C haos Space Marines  with the Mark of Nurgle are likely to become  Plague Marines, gifted with almost unstoppable resilience by the Plague Lord. These champions are compelled to constantly spread  Nurgle's Rot  throughout the galaxy.

Source

 * Codex: Chaos Space Marines (3rd Edition, Revised Codex)
 * Codex: Chaos Space Marines (4th Edition)