Bertha Mason's insanity is neurosyphilis.
Bertha was noted as cheating and drinking and generally being a rather immoral person, even before she lost her mind. And her mother was noted as having gone insane too. She (and likely her mother) have/had neurosyphilis which is known to cause, among other things, sudden personality changes, changes in mental stability, irritability, and mood disturbances. Neurosyphilis also tends to have a muscle weakness symptom, but not all people show all symptoms all the time, so that can be explained why she's still very physically strong. Also, Mr Rochester goes into some detail blaming her insanity on bad genes and the fact that she is mixed race. When syphilis is passed down like that, it can look like a genetic disease.Another theory is that Bertha has Huntington's chorea
, which is also passed down. Rochester knows her family has 'idiots and maniacs through three generations'. Bertha's mother was a Jamaican Creole, "Creole" in this context meaning mixed race, probably with some African heritage; there is a form of Huntington's that is more common among people of African descent.
Jane is on the autism spectrum, as are several other people in her life.
Several contemporary literary critics have suggested this. Jane's introversion and social awkwardness, her alternation between extreme reserve and "unfeminine" boldness, her high intelligence and "unchildlike" manner as a little girl, and her talent as an artist (which she sometimes uses rather than words to process her feelings) can all be seen as traits of high-functioning autism. The fact that other characters repeatedly call her "strange" and "queer" only reinforces this, as does her self-aware suggestion that Rochester views her as "an automaton." St. John Rivers (with his repressed emotions and life of logic and religious obsession), Eliza Reed (who lives by a rigid "system," divides each day into "sections," and withdraws from society to be a nun), and Helen Burns (an introverted bookworm, painfully disorganized and prone to daydreaming, yet deeply thoughtful, intelligent and mature for her age) also arguably show traits of the spectrum, and/or of ADHD on Helen's part.