Some sufferers of fibromyalgia have postulated that Florence Nightingale was also a sufferer, based on (I believe) accounts of her symptoms in her diary. I can't find much serious research backing this particular example up, but it does seem that the application of modern medical knowledge to contemporary accounts of historical figures and their illnesses is reasonably common. How accurate is this? And how does it change our understanding of the historical figures involved?
Even in situations where an accurate diagnosis can be made, the utility of this kind of diagnosis can be questionable.
All diseases are, to some extent, culture-bound. How we interpret and feel about symptoms and treatments, and how those feelings and interpretations inform our actions are directly tied to what we believe to be happening. For example, there was a time and place in which finding suppurating sores on one's skin would be a relief (it means that the bad stuff is coming out!).
While there's perhaps some ways that knowing what a historical figure "really" was suffering from might help us better understand the past, it can also get in the way of understanding it if you apply it too rigidly.
Sometimes it works best to accept that what someone was suffering from was exactly what they and their contemporaries believed it to be, even if it's not a real thing anymore. If there's no context to understand, say, fibromyalgia, in the 19th century, declaring that a case of hysteria was 'really' fybromyalgia might obscure more about lived experience and context than it reveals.