Remember that gods aren’t responsible for doing things but for documenting things. So there are little gods of diseases whose job is to tally up how many people die due to a particular outbreak of plague but they don’t cause the plague. That’s caused by imbalances of Essence as a result of not following proper rituals that just happen to look like modern sanitation.
Now the gods, even the least gods, can do stuff like cause and cure plagues. It’s part of their mandate to maintain Creation after all. So if some raksha asshole (but I repeat myself) goes around fucking up Fate and causing plagues without the imbalances then the least gods can step in to fix it, usually in response to human efforts… ie, thaumaturgy, mostly via the use of the Medicine ability.
It’s like the opposite of 40k, where the process of tuning up an engine has been turned into religious ritual. In Creation, religious rituals have been turned into mechanical processes.