Nurgle isn't about disease itself, but more of the very concept of a perversion of the neverending cycle of life, the idea of rot and decay without passing, disease without end. By embracing the rot, they conquer death itself, but in perverting it, they reject the cycle and change that comes with it, which is why they are naturally opposed to Tzeetch.

Nurgle's contagion is also often literal mindcontrol, like people literally just get infected with a plague that takes control of them. The rest of it is someone wallowing in despair, particularly when afflicted with magic diseases, until they become blackpilled and start reveling in their stagnation and decay and trying to spread their literal brainrot to others. And also the usual Chaos hyper-fascism shit of people seeking power and self-actualization through spreading horror and worshipping the ruinous powers, and just rolling the extra icky god from the "I'm going to commit abominations against reality for fun and profit" results chart on their rituals.

It really does make a lot of sense when you look at actual reactionaries, and at its core does make for pretty good writing casting these pure concepts of fascism and related reaction as the foils to a brutal, corrupt, and stagnating empire that was built according to an inscrutable plan to play with the line where that remains the lesser evil compared to Chaos. Yet another way the Imperium is fundamentally a liberal autocracy, except unlike real liberals they actually materially crack down on subversion from their right instead of only from their left.