Wild assed guess, but I’m personally going to bet on, “Poor Health.”
I think that in the grand old days, a lot of people were living in unclean surroundings, and drinking unfiltered water that probably had anything and everything in it from sewage and lead, to rotting corpses. Women generally weren’t active and either lived such protected lives as to faint at anything out of the ordinary or more probably simply knew that it was fashionable to seem so, but either way probably they still weren’t terribly set up to handle a lot of stress.* And also, with the state of medicine and medical labelling, probably a lot of people were taking semi-poisonous prescriptions or over-the-counter drugs (like opium.)
Added to that, simply they didn’t even know what was wrong with people a lot of the time, so given that, “brain fever” could be one of any number of things.
And finally, I think that what may have been of much rarer occurence (fainting, “brain fever”, etc.) is more likely to occur in popular literature than real life.
- I would also somewhat wonder when the height of corset-tightening was. I think I recall reading that these could be rather harmful. I’m pretty sure I’ve seen a picture that showed how such a corset had litterally compacted a woman’s body so that her inner organs even had been squished up and down permanently inside her torso.