Do 你 think sickness in the 阿凡达 world is caused 由 spirits?

That's a good 问题 and a good theroy.

I mean it's possable. I personally don't think so I mean we saw Sokka and Katara get sick in season one just for being out in a storm (a natural cause) none of them had an spiritual problems (that I can remember).

I think Zuko's was the result of stress. Real life science has proven that extreme mental conflict and stress can induce physical illness, and I think that's what happened to Zuko. And his dragon dreams were the product of his conflicted state.