Well, the grand majority of Nurgle's plagues only work on living organisms with an anatomy.
Daemons are neither.
He does have atleast one plague, Nurgle's Rot, that infects the soul, not the body.
It seems reasonable that he must have some anti-daemon plauges, however, when the forces of Nurgle work in tandem with another God, his "gifts" are with held to his own children.
As loathsome as it may be to Nurgle, he can indeed cure every illness conievable, and it is not uncommon for those suffering from sickness to pray to Nurgle for relief.
How often he chooses to dispence such relief has never been mentioned, but it is said that cries of agony and relief, even curses to his name from plague victums are like the laughter of loving children to him.