@dirtychai The symptoms don't match scurvy at all, so it would be strange if it was. Plus going by their level of medicine scurvy would have been long since figured out and sailors would know how to treat it. Syphilis would need an anti-biotic to treat properly which wouldn't be discovered yet, mercury treatment did work but then you would die of mercury poison which was just as bad if not worse. I assume it's mercury poisoning but why they wouldn't just outright say that I don't know. Of course it could be a long long list of other venerial diseases, or just general diseases too someone would get traveling to another continent.
Read Cursed Princess Club, it's finished and fantastic.