I was reading “The Plantagenets - The kings who made England” and in one instance it is mentioned Geoffrey Plantagenet died of a fever in 1151 at the age of 38. Do we know what sort of illnesses would have been described as fever in the Middle Ages? Are there illnesses that were more deadly then due the lack of modern medicine?
The problem with fever as a symptom is that it's one of the body's go-to responses for fighting off an infection. You can get a fever from anything from the common cold or flu (which don't forget, can still kill 10-20,000 people a year in the UK and in one bad year in 2014/5 killed almost 30,000 [Public Health England; Surveillance of influenza and other respiratory viruses in the UK, Winter 2018 to 2019]) to septicemia or sepsis from an infected wound, to meningitis, to a simple stomach bug.
Medieval medical treatments were often actually surprisingly effective, and were on the whole far more evidence-based than pop-history might expect, but its treatments were often based on the treatment of symptoms rather than, necessarily, the cure of the disease. That is to say, there are plenty of cures fora congested cough and persistent headache, say, but nothing that might clear up the tuberculosis that was actually causing those symptoms. Of course, if the majority of your patients just have a chest infection or sinusitis, then those treatments are going to be recorded, broadly speaking, as being at least to some extent effective.
So it was with fevers. Medical texts such as the 9th Century Bald's Leechbook contain treatments for a fever which modern science suggest should have some efficacy:
For fever again, drink a lot of betony and eat three slices; Again drink wormwood, corncockle, betony, marsh mallow, water mint, ragwort, the clove-bearing lesser celandine and white horehound in clear ale. Drink for thirty days.
It was understood that fever could a symptom of an infected wound or injury, a stomach infection or liver disease, and treatment of those ailments also often contain a provision for treatment of the concurrent fever, but in many cases, without modern diagnostic knowledge, the actual cause of a symptomatic fever could be a complete mystery, and while the symptoms could perhaps be lessened, the underlying condition may well go untreated and unidentified. In cases of fever, therefore, a physician might have to hope that the fever was a symptom of a relatively minor infection or passing bug, but if it was a more serious illness that couldn't yet be identified, then the fever itself might appear as the cause of death.