The Shulchan Aruch in Orach Chaim 173:2 writes:
בין בשר לדגים חובה ליטול משום דקשה לדבר אחר וחמירא סכנתא מאיסורא
There is an obligation to wash one's hands between fish and meat because it causes "דבר אחר" and we are more strict about things that are life threatening than things that are forbidden.
The source of this is a Gemara in Pesachim 76b which states:
תני רב כהנא בריה דרב חיננא סבא פת שאפאה עם צלי בתנור אסור לאכלה בכותחא ההיא ביניתא דאיטווא בהדי בישרא אסרה רבא מפרזיקיא למיכליה בכותחא מר בר רב אשי אמר אפילו במילחא נמי אסורה משום דקשיא לריחא ולדבר אחר:
Rav Kahana, son of Rav Ḥinnana the Elder, teaches: In the case of bread that one baked together with roasting meat in the oven, it is prohibited to eat the bread with kutaḥ, which contains milk, because the bread absorbs some of the meat’s aroma. The Gemara relates: There was a certain fish that was roasted together with meat, Rava of Parzikiyya prohibited it from being eaten with kutaḥ, due to the meat flavor absorbed in the fish. Mar bar Rav Ashi said: Even to merely eat it with salt is also prohibited because meat that is roasted or cooked with fish is bad for odor, meaning it causes bad breath, and for something else. Therefore, one should avoid eating it due to the danger involved.
Rashi ad loc. explains that the "דבר אחר" is Tzara'as, a spiritual skin-malady that hasn't existed in hundreds, if not thousands, of years. The Shulchan Aruch writes this as well in Yoreh De'ah 116:2.
We have no source that I'm aware of that this Tzara'as was dangerous. If so, why does the Shulchan Aruch as well as many other Poskim, call it life threatening?
(Note that Tzara'as is not leprosy even though it is commonly mistranslated as that. See here, here, here, and here for several of the hundreds of sources that say this. Also see שו"ת תורת חסד אבן העזר סימן ה סוף אות ה' that is explicit that the Tzara'as caused by eating fish and meat together is not a physical malady. Also, note that I am not looking for opinions that argue on the Shulchan Aruch and say it's not a Sakana. I've seen the Magen Avraham, Chassam Sofer, etc. I'm merely looking to understand the Shulchan Aruch and those that agree with him. I'm aware that the Divrei Malkiel says he doesn't mean it. That's a Dochek and certainly doesn't fit with all of the Poskim who actually treat it as more Machmir than Issur.)
Another side question that bothers me is being that Tzara'as is a spiritual malady that comes from behaving inappropriately (such as by speaking Lashon Harah or being miserly), not a physical one, how can eating fish and meat together cause it?