Pyrotherapy. The idea of fighting untreatable (at the time) illnesses by inducing a fever in the patient. There's actually a Wikipedia article about it: https://en.wikipedia.org/wiki/Pyrotherapy As in deliberately infect a syphilis patient with malaria, which can give you a very high fever. The first time I heard about that one was in Neal Stephenson's Baroque Cycle, turns out it was one of the non-fictional parts of the books (which were a mix of historical facts and fiction).

Quote: "In general, the body temperature was maintained at 41 °C (105 °F).[1] Many diseases were treated by this method in the first half of the 20th century."

The malaria variant was not the common variant, that was saved for extreme cases, apparently (e.g. Syphilis). Mostly it was hot baths and the like.

I, for one, will stick to my near-daily sauna sessions.

Oh! Today's a good day for sauna.