Working with 18th century burial registers that list causes of death, many thousand km away from England, in 'wild' EC Europe. Have also had the pleasure of studying Eng lit (mostly medieval, but some Victorian too). In practice, causes of death listed in various sources depended on the recording instance's knowledge of disease, the deceased individual, and on the documentary context. Although by the mid-19th cent medical knowledge had advanced significantly since the mid-eighteenth, symptomatologic descriptors of diseases -their outward signs - were generally still recorded in burial registers, instead of very clearly branded infectious diseases (cholera, pox, etc.) You therefore often encounter 'brain fever','headache','convulsions','hectic fever',etc. Dying of old age is generally described as 'frailty or weakness'. Those who suffered from a heart attack or similar fell to 'apoplexia' or the like. Surprisingly, some very visible kind of diseases that probably left sores (but not syphilis) are described as 'canker'. But this is one context. Brain fever as described by Brönte could very well be akin to meningitis, weakness and frailty due to anemia, etc. Anything the tiniest bit infectious wreaked havoc on the day to day lives of people, well into early 20th cent (penicillin). While infant and child mortality were the highest (could reach 1 in 5 children dying before 1y old), young people were not spared from pneumonia, etc and the likes. A fever might very easily have killed an otherwise healthy 20y old in days, if it was associated with other complications (pneumonia, vomiting, etc.) On my phone now, but you can look for the international classification of diseases, which is what we use in historical demography to help figure out what actual ilnesses hide behind our generally unclear and symptomatic descriptions. It should also have a section on historical terms for diseases, or historical definitions/descriptions of diseases. This is taking a new historicist approach to Victorian lit, which you might disagree with. Nevertheless, the Victorian understanding of diseases (particularly melancholy, and other such complaints that emphasized the mind or heart's effect on the physical well being of the.body) certainly underscored the Bröntes' portrayal of their characters. Will suggest some readings as soon as I get to a computer. Wonderful that you're interested in this!

Edit: Have returned with sources:

Two good articles on reporting of causes of death and the development of the ICD 10 (the international disease classification I had mentioned, whose exact name escaped me): *Williams, N. The Reporting and Classification of Causes of Death in Mid-Nineteenth-Century England.The Example of Sheffield, In Historical Methods (1996), issue 2. That particular issue is actually entirely devoted to historical causes of death, so it might be of interest. *Douglas L. Anderton and Susan Hautaniemi Leonard, Grammars of Death: An Analysis of Nineteenth-Century Literal Causes of Death from the Age of Miasmas to Germ Theory, In: Social Science History, Vol. 28, No. 1 (Spring, 2004), pp. 111-143

*Then you also have some recent studies done on melancholia - depressive psychosis and morbidity: The morbidity and mortality linked to melancholia: two cohorts compared, 1875-1924 and 1995-2005

*Finally, on heartbreak as a cause of death in Victorian literature: Blair, Kristie, 'Proved on the Pulses'. Heart Disease in Victorian literature, 1830 - 1860, in Framing and Imagining Disease in Cultural History, eds. G. Rousseau et al., Palgrave MacMillan 2003.

The Burden of Disease and the Changing Task of Medicine is an excellent article that addresses both the changing prevalence of disease as well as the changing conception of disease across time. The descriptors we use for disease have evolved along with our understanding of disease and this change across time makes it challenging to know precisely what is meant by some medical terms. The second figure in the article linked above is a death registry for the town of Boston, 1811. It lists familiar causes of death like drowning alongside causes of death that are difficult to understand from our modern perspective, such as 'white swelling'. One thing that is very clear from this 1811 registry is that infectious diseases were a much more common cause of death in the past than they are today. Consumption (a syndrome that we currently understand to be the most advanced state of tuberculosis) is responsible for the 221 of 942 total deaths, and fevers are responsible for 116 of the total. This interactive table from the article shows the top 10 causes of death in the United States from 1900 to present, and while it does not stretch back to the time period you are discussing it is a good illustration of the degree to which infectious diseases have historically been among the most common causes of death.

Brain fever is among the terms that are difficult to understand from our modern perspective. Fever itself may have had a broader meaning than elevated temperature. That being said we can look to other sources of the time to tell us that 'brain fever' referred, at least some of the time, to what we now think of meningoencephalitis. (To choose another literary source) In Laura Ingalls Wilder's By the Shores of Silver Lake she describes the illness and subsequent blindness of her sister Mary. In the book Mary's blindness is (mis)attributed to scarlet fever. However examination of primary source material indicates that Mary's illness was referred to as 'brain fever', 'spinal sickness' and 'stroke'. Through the modern medical lens she likely suffered from some form of meningoencephalitis.

Illness such as 'weakness' and 'apoplexy' are also poorly understood in modern medical terms and likely describe a wide variety of what we currently think of as medical conditions. This article from 1812 discusses the dissection of 2 patients who died of apoplexy. The discussion (the final section of the paper) makes it clear that apoplexy is generally thought of as collapse followed shortly afterwards by death and is known to be a brain disease, and that specific causes such as hemorrhage of the brain were known. The discussion (and this follow-up letter) discuss the possibility of causes of apoplexy other than pathology of the brain. This is the beginning of a discussion that leads to our current knowledge of causes of sudden death, from the anatomic to the molecular.

As to the issue of whether Bronte would have known young people who died (vs. this being a romantic notion) reference to decennial life tables shows that death at young age was more common in the past than it is at present. More specifically the Bronte family had personal experience of early death, both the death of their mother at a young age as well as their own deaths. Tuberculosis is thought to be the cause of death of many members of the Bronte family including Emily in 1848 not long after she completed Wuthering Heights. Catherine is often given as an example of a literary character suffering from tuberculosis, and given the temporal relationship between the deaths of the Brontes and the writing of the novel it is reasonable to think that Catherine goes to live with the Linton's because she too is suffering from tuberculosis.

You have asked an interesting and complex question that links the medical and literary worlds. In summary it would have been much more common for people to die at young age, and much more common to die of infectious diseases in particular circa 1845. The Bronte's had both personal experience of premature death as well as personal experience of tuberculosis. The language surrounding death in older literature seems vague and even 'flowery' but it is clear that it is at least in part related to differing conceptual frameworks around illness and death than we currently have.

I hope that is helpful, I have never answered a question in this forum before, but answered this particular one as a physician with an interest in medical history and representations of illness in art and literature. Please let me know if this answer requires revision or if you feel it is not sufficiently detailed for this forum.

(minor edit for clarity)