#the entire fantasyworld would collapse the moment he dies of old age

[richardsphere]

OH i know this one. Your first step is to figure out whether your fantasy setting is more western or eastern based. This has a bunch of nuanced ways to discuss, but the most important one for the purposes of modern medicine is: do the locals drink tea, or do they drink wine. If they're a tea-based society, their primary storage vessels will be earthenware. This'll be a problem. If you want modern medicine you need to first popularise wine, this'll create the supply/demand impetus for the locals to create a proper glass-blowing industry. You cannot have modern medicine without having access to a good glass-blowing industry. Because glass is chemically inert it is the one material you absolutely need for your any proper lab-setting. You cannot store your medicines in reactive containers after all, nor can you safely make medicine in reactive metal pots. Even if you are already in a wine-based society with a functional glass-blowing industry, you still need to get some glassworkers to switch into glass-cutting from glass-blowing though. This to gain access to microscopes and glasses. The glasses especially matter a lot more then you might think, the main source of scientist retirement is age-related eye-deteriation. Prescription Lenses earn every scientist an aditional 5-10 years in which to work. From thereon, you can expect modern medicine to slowly emerge as a result. (alongside modern chemistry, remember to warn them about the leaded gasoline and PFAs)