Like, is it just an allegory for diesease and pestilince that tends to grow in swampy, damp, dark places or is there actually a type of body of liquid that would be poisonous to tread through and not by injesting or infecting through an open wound? 🤔
The only places I could think of are not swamps and are all a result of horrible pollution like Lake Karachay or Rio Tinto. I think you are correct that the trope of the poisonous swamp stems from the fact that swamps are just not very nice places to be in as a human.