Research: Done some poking around on Google looking for things like "antibacterial plants," "medieval illness." Looked through the tags here, particularly "illnesses to order"
Setting: medievaloid fantasy world. The specific place is a town near a marsh, with a fairly mild/damp climate.
I need two diseases that can easily be mistaken for one another through external symptoms, but can be told apart through a blood test (a quirk of my MC's magic lets her do things like this). They need to strike fairly quickly or have a long mild stage before suddenly worsening--functional to laid flat within a day. They need to be fatal if untreated. Ideally, the illness-whose-treatment-kills is, will be fairly well known, and definitely better known than the other illness.
Here's the kicker: the treatment for one needs to be a death sentence for someone who has the other, or at least make the other illness significantly worse. And the treatment needs to be available through the native plants of the area. It doesn't matter if the treatment would have been understood or known during the medieval era, and it doesn't matter if whatever plant(s) are used have to be processed in some unlikely way. It just has to work.
My thought right now is typhus, treated with the glycyrrhizin in mullein. I have absolutely no idea what typhus could be mistaken for. I'm happy to scrap this if something else will work instead, or if it's intensely faulty.
If there's nothing, I can handwave the whole thing and make up a whole bunch of names, but I'd really prefer not to.