In some eighteenth-century era novels, a character will be described as being diagnosed with “brain fever” (Catherine Earnshaw in “Wuthering Heights” is an example). What exactly is this condition?
And further on the subject of fevers in general…other novels will refer to a character of same era as “dying of the fever.” What fever were they talking about? Malaria? Scarlet fever? Or some undiagnosed condition that simply produced a fever?
Prior to the 1870’s or so if IIRC germ theory did not even exist in any meaningful form. Many diseases could easily manifest primarily as fevers, especially something like viral meningitis.
Take your pick-
Before antibiotics and an understanding of the importance of aseptic techniques any minor infection could turn into overwhelming sepsis fairly easily.
Viral or bacterial meningitis, encephalitis, malaria, diptheria, tetanus, simple bacterial septicaemia (from say a UTI, chest infection or infected cut), measles, TB, scarlet fever, rheumatic fever. Any of them could present as a fever and could proceed to kill you.
Plus of course other things like strokes and brain haemorrhages which might present as an altered mental state proceeding to death.
Someone with fevers and night sweats might also have been dying from lymphoma or leukaemia.
Diabetic ketoacidosis would have killed you pretty quickly too, before the discovery that insulin sorted it out.
People with high temperatures can develop delerium or seizures, especially if the fever isn’t detected early or brought down quickly. Anyone delerious, confused or having seizures might be thought to be in the throes of a “brain fever”, even if the real problem was a kidney infection that was running unchecked.
If you have seizures or a high temperature for a long time, you can end up with irreversible brain damage and death, again something which might appear to be a primary problem with the brain, while actually it was due to hypoxia during the seizures or simple hyperthermic cell damage.