Parasites can be spread unintentionally by NPCs, players, friendly monsters, swamps, and so on. Poison is spread delibrately, unless someone was brainwashed, in which case it is still delibrate since the brainwasher wants the brainwashed to do something.
As for plagues... Well, they should only come up only once in a while, and when they come, the healers will have a field day healing patients.
Parasites can add to gameplay. Healers or self-healers may find healing similar to a mini-game, or they should if only for the sake of gaming.
Will add more later.