One of the unresolved issues in medical halacha is how to apply laws that were fixed at a time when medical science was different than it is now. An example of this is- when one is permitted to violate Shabbos for a fever? A life-threatening fever justifies and sets aside Shabbos prohibition, as does any other life threatening illness. How is a dangerous fever defined?
Until farily recently what we now call "symptoms' were considered diseases. For example, a patient suffered from diarrhea, or fever, or fainting spells. Vestiges of this mentality survive in medicine to this day in the names of certain diseases. We speak, for example, of Rehumatic Fever, or Yellow Fever whereas doctors now see these as conditions caused by infections by specific micro-organisms. In the current medical paradigm, it is the dangerousness of a disease that should determine whetheer Shabbos is violated, not the height of the fever. A low fever casued by a serious illness should be treated with greater seriousness than a high fever caused by a benign illness. Is this how halacha sees it?
The underlying diagnosis of what is causing fever may or may not be of a halachic significance. Ktzos Hashulchan 138 (p.99) writes that a temperature of 38 degrees Celsius allows taking medications on Shabbos. R. Moshe Feinstein in Igros Moshe, Orach Chaim 129 wrote that a temperature of 102 degrees Farenheit (and even 101 degrees, if the patient insists) allows the suspension of Shabbos prohibitions. These poskim do not incorporate the underlying diagnosis into this calculation, probably because classical sources do not do so either. Some however suggest that the underlying diagnosis should also be taken into account when assessing the degree of fever for the purposes of Shabbos violation (The Halachic Guide to Medical Practice on Shabbos, by Michael Chizkiah, Targum Press, 2005, p.199, fn 4). It is my impression that most poskim in our time and place would take the underlying diagnosis into account in regard to this question.