This M.Y. answer lists a prohibition of eating fish and meat together because of sakana - it was dangerous.
What was the danger at the time this prohibition was instituted?
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It can cause leprosy. I heard this at a shiur (if I recall correctly, given by Rabbi Michoel Stern), but I found this online: http://www.myjewishlearning.com/article/ask-the-expert-meat-and-fish/
Edit: Pesachim 76b
A fish was roasted [i.e., baked] together with meat, [whereupon] Raba of Parzikia forbade it to be eaten with kutah. Mar b. R. Ashi said: Even with salt too it is forbidden, because it is harmful to [one's] smell and in respect of ‘something else'.
Rashi explains 'something else' as tzora'as (i.e. leprosy).