the way I have always interpreted the disease and fire things Jek, is that the reason that say Silvanus opposes Talona is not that they represent disease, but that given their way, they would upset the balance. Talona would see everything wither and rot with disease, which Silvanus would oppose because it is taking something that is a part of nature and taking it too far, it's why he's NN, he would theoretically be just as opposed to say.. someone curing all disease so that everyone could live forever, though that may be seen as harsh. Malar is a perfect example of this, Talos as well.

As for the difference between druid and cleric, a cleric is a priest, associated with a church whose goal is the spreading of their gods message and furthering the church. a Druid is more holistic their first priority is the protection of nature, through this and the actions they take they serve to further the agenda of their god.

I suppose the way I would think of it is, a druid serves a god (somewhat indirectly) through their service to nature

a cleric of a natural god, serves nature indirectly , through their service to their god.

its why you can have a cleric of any god, but a druid of only some.

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Happiness is not a goal; it is a by-product.
-Eleanor Roosevelt.