Nurgle is a very very very busy man! He's making new plagues every 0.2 seconds and it's genuinely a scientific nightmare to try and patch them out or heal them up because they will just change or new ones will come up. Even if you picked a setting like Star Wars and threw every single Nurgle troop he has avaiable in it they could not possibly fight them all off. It's also hard to research plagues that kill you in 3 seconds. Nurgle makes so many plagues at the same time he needs his generals to carry around a little notebook detailing and remembering the names of every sinlge one even if they are just accidentally created. The general lack of healthcare in the 42nd millenium specially in but not exclusive to the imperium also feeds Nurgle and it's something most authorities in the imperium will do nothing over.