Going back to your infectious diseases for worldbuilding: what are some of the best and worst examples you've seen in fiction. Beyond that, what factors impact how scientific and medical knowledge and practice progress in a fictional society?
Oh man, there are a lot of terrible examples, mostly of the "this plague kills everybody and fast and if you're exposed to it, you have it, and it works like magic but is supposedly scientific" variety (this is actually more common in scifi than fantasy). In fantasy what I've noticed is that there are just so many authors who completely disregard the concept of infectious diseases completely. It's just absent. Battlefield wounds never get infected, and nobody spends the days and weeks following worrying about death from infection. Nobody ever talks about siblings or children who died before their first birthday because they got a fever. People who die from childbirth die from bloodloss, not the much more common childbed fever. Nobody worries about STIs. Fantasy Rome or Renaissance Italy might have the plague, but they don't have the perpetual scurge of malaria. Armies never miss their climactic battle because everybody is down with typhus or dysentery.
Obviously this lack of disease is because it's not what the authors want to focus on, but for the ones who do, or for the ones who just want to have it in the background, I'm making some videos to help.