Help finding a plague/ disease to use?
So my MC is a well established and powerful (Medieval-esque fantasy) Lady who becomes this huge patron to the sciences, and funds tons of research on stuff like diseases/ medical advancements. In short, war breaks out and everything goes absolutely terrible for her, to the point where she looses enough of her morals and mercy and decides biological terrorism is the last resort, but still has enough mercy to not want to let it spread into her lands and hurt innocents (wants to just unleash it in war camps and enemey keeps).
Thus my question, I would like to find a real life disease/ plauge/ whatever to reference that is:
Doesn't require antibiotics to be cured (they dont exists), vaccines and other treatments are great though
Contagious between humans/ their water/ food, or just highly contagious in general
Thrives in warm areas and dies off in any cold regions
Probably doesnt exist, but if it would primarily affect adults over children, or affect the rich over the poor, that would be a huge bonus.
A quick and high probability of death
Right now I am considering Smallpox (because her lands could be largely vaccinated with cowpox) or Cholera (because treatment is largely just better fluids), but I would love to hear any other reccomendations or thoughts you think I should reference/ consider!