Board Thread:Warhammer 40k General Discussion/@comment-192.226.225.70-20131002223609/@comment-5520074-20131003100008

From the 6th Ed. Chaos Marine codex, and I quote:

"Plague Marines are Chaos Space Marines who have sworn themselves to Nurgle, the Chaos God of pestilence and entropy..."

"Plague Marines have disgusting, rotted bodies that stink of decay. The putrescent slime that oozes from their many weeping sores corrodes their armour and boils away their skin, but despite their horrific disfigurements, the Plague Marines are fearsome warriors indeed. Their rotting brains are inured to the agony of bodily corruption, making them all but immune to the pain or debilitation caused by battle wounds."

If you say tl;dr, they are blessed by Nurgle. Nurgle's gifts involve rotting and decaying anything he blesses.