You know, between this, Veekie's suggestion that the plague itself might have come from a horse-based strain, and the fact that we have an open spiritual slot I feel like there is a potential lose-lose situation in the future (assuming we survive that long, of course)
Now pardon me for being pessimistic, but there are a few outcomes I can see happening:
-Civ dies from the plague, knowledge of where the plague came from is pointless.
-Civ survives from the plague, source of the plague is known and comes from horses: potential spiritual value / debilitating belief against using horses: gives us a heavily skewed and inflexible army composition
-Civ survives from the plague, source of the plague is unknown: potential spiritual value investing in superstition and away from Philosopher Kings, seeing as proto-empiricism couldn't find the answer.
Additionally, I feel like people's suggestions that the Ymmri are 18th century in terms of urbanisation and bringing up making attenuated vaccines without the help of microscopes, precise thermometers and volumetric measurements is a bit... frustrating. I know we like to stroke our egos and suggest that we created a "super-civ" that is centuries ahead of its time. Heck, it may be even true with regard to certain tech fields. However, through these tech advances we've also inadvertently caused our culture to develop a bit haphazardly. While this may be a bit of a hyperbole, sometimes I feel like we are looking at what giving an Iron-age civilisation (with their respective morals, norms, and culture) bits and pieces of advanced technology will do. While some good may come from it, there are also a slew of problems such as our relative ineptitude at espionage (why look outside when we clearly have the best tech?), the persistence of inter-clan violence (having been supplanted somewhat by the different social classes now, but in a way it's become just another extension of the clan violence), and the effects of a (relatively) sprawling empire (we have the people and power to take it, but we don't yet have the communication technology to maintain it, it seems).
Would these problems arise if we didn't focus on tech and agriculture? Maybe. There would likely be some similar problems, along with other ones as well, but I just wanted to point out that just because we gave cavemen steel tools and guns, it does not mean that they will somehow magically adopt Enlightenment/Modernist/Post-modernist Era thinking. That takes centuries/millenia of struggles and conflict (like this plague right now!
) to cultivate.
TL;DR: Ymmri is still an Iron-Age Civ, don't expect us to leap ahead on the whole just because we have a few anachronistic techs.