system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Seriously, how have the Death Korps of Krieg not fallen to Nurgle yet?

Their entire specialty is endless, grinding, miserable wars in chemically soaked, dead wastelands. They sound like a perfect canidate for corruption.
gpt
How do Chaos God's fight?

The corruption of Chaos God is not some disease. It's more of propaganda. Enough chaos supporters come together. Perform dark rituals, open portals. And boom, you have chaos infestation. Which only after spreads actually physical corruption.

Now after this happens the forces of chaos generally fight to overwhelm enemy, and it usually devolves into fights of attrition. 

Now what does Krieg excell at? Zealousness and siege (attrition) warfare. Not only they are not prone to corruption, they are zealously devoted to the emperor. To the point they will often hurl themselves into suicide situations. They do not fear of death, they embrace it. Chaos have no power over them. The usuall tactics don't work.

Members of Krieg are zealous. So chaos cannot persuade them. They are fearless, so chaos cannot scare them. Chaos cannot beat them into submission. Because they excel at attrition warfare. Chaos cannot infect them with dissease, toxicity, nothing. Nothing works against them.

tldr: Their existence is miserable enough, so Chaos cannot do to them, for what they are not prepared for.