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...
A race that are always born as twins and only one recieves a soul. The other is soulless therefore without pity, shame, or guilt. The evil race. Can't tell them apart by looking at them.