Animals have parasites and diseases, so do plants, there are even bacteria that cann dissolve oil spills or plastic or what have you - why couldn't there be a disease living off magic - a magic item could be infected, losing power and speading the plague to other equipment it contacts, while a magic user would slowly lose his powers and contaminate all he works with or casts a spell at...
"...the Ilthian mountains. A craggie masse of rock rysing from the Ilth'n plaines. The waters whych springe from it are ful of godeness and fortyfie those who drynk them [+1 STR]. Alas the vyle beasts resydent in these hills also bathe in these waters, and in the doing gain great strength. Foes mortallie wounded have bene known to flee, onlie to return, revytalised houres later..." - Chronicler Eamusil, Mondopedia, Vol XV (The Lands of Sylmen)