1: People specialize in disease?
2: There's very few disease effects open to PCs. The Plague Bringer Alchemist archetype, the obscure Pestilence Bloodline and four spells (all [evil]), one of which causes malnutrition instead of a disease, are all I can find.
3: People who fight PCs rarely live to tell the tale.
4: Just say magical effects that cause disease are a different strain each use.