Spirits are essential to just about everything in Roshar, the world of Brandon Sanderson’s The Way of Kings. When it’s raining, rain spirits come out to splash in puddles. When the wind blows, ghostly, luminescent, mischievous women ride the zephyrs. Fire dances, not figuratively, but literally. And it’s not just physical phenomena, either, that attract spirits in this world: glorious deeds, perfectly logical arguments, and especially creative or beautiful artwork also have spirits associated with them. These spirits are called spren, and you might think they would suggest a world in which the scientific mind could easily shut itself off entirely. After all, it’s hard to imagine a setting where it would be easier or more tempting to answer any new problem with “a spren did it,” and then stop thinking.

And yet, the people of Roshar saw those little spirits and used them to formulate the germ theory of disease hundreds of years early.

Introducing rotspren:

“Keep that wound clean. We don’t want to attract any rotspren. Let me know if you see any. They are small and red, like tiny insects.”

Cute little things, aren’t they? Like ladybugs that give you gangrene! Rotspren are drawn to open wounds and decaying corpses. Wounds that draw rotspren will quickly become inflamed and infected, causing illness and death if treated poorly. However, rotspren may be small, but they’re still visible, and this has given the people of Roshar the chance to learn a lot more about germs than they should have.

“Hands,” Lirin said, not turning away from gathering his tools. Kal sighed, hopping off his stool and hurrying over to the basin of warm, soapy water by the door. “Why does it matter?” He wanted to be at work, helping Sani. “Wisdom of the Heralds,” Lirin said absently, repeating a lecture he’d given many times before. “Deathspren and rotspren hate water. It will keep them away.”

When you can actually see germs crawling all over everything, it becomes much easier to realize some basic things about hygiene: If you wash your hands, you aren’t covered in deadly ladybugs that make wounds rot. A strong incentive, no? It gets even better: the visibility of rotspren has massively sped up their discovery of antiseptics. The people of Roshar have found multiple naturally occurring compounds that frighten away rotspren: larmic mucus, knobweed sap, and lister’s oil. And, although it’s not made explicit, I imagine other practices that caused a lot of real life trouble weren’t a problem in Roshar. (It’s hard to imagine people looking at corpses that were crawling with rotspren and saying “Yes, let us put these corpses next to our drinking water.”)

To review briefly: In our world the transmission of diseases remained largely mysterious until the invention of the microscope. Although Girolamo Fracastoro invented the contagion theory of disease in 1530, and suggested that tiny particles that he called spores transmitted infections and caused epidemics, his theory was not widely adopted, and science didn’t formally recognize microorganisms as disease vectors until Louis Pasteur’s experiments between 1860 and 1864. Only when he could show people bacteria under a microscope would they begin to believe in germs. Before that the predominant theory of disease, at least among the theories that didn’t revolve entirely around acts of God, was the miasma theory, which proclaimed that sickness spread through bad smells. Pasteur’s experimental demonstrations that microorganisms caused disease paved the way for Joseph Lister to massively improve sanitation in hospitals by inventing antiseptics. He also invented Listerine, although he intended it to be used to clean floors and cure gonorrhea, not to clean out your mouth before a date.

Is this the same Joseph Lister who discovered lister’s oil in the world of The Stormlight Archive? Probably not, although it’s possible that Lister represents the Platonic ideal of inventing sanitation in Sanderson’s mind. Either way, his fantasy innovations have brought medical science to the levels of the dawn of the modern era, while most of the rest of Roshar’s technology is still medieval at best. Surgeons are extremely highly trained, maintain clean operating rooms, use antiseptic, have primitive anaesthetic, and seem to save far more people than they lose, even when they have to amputate limbs. None of this was possible in real life until a time when we had guns, trains, steamboats, and light blubs. People in Alethkar, the main nation in The Way of Kings, are still riding horses, traveling long distances by carriage, and swinging swords at each other, albeit massive ancient swords called Shardblades that cut through stone like paper and burn the souls right out of a person’s body. I’m not sure they even have crossbows. There is a certain amount of disconnect here.

By using the magical elements of his world to screw around with its tech level, Sanderson reminds me of what I love about fantasy, and speculative fiction more generally. When an author makes a new world, they get to set new rules, rules that can do so much more than just set the terms of magical battle. Rotspren change the rules of medicine just as much as Shardblades change the rules of warfare. After introducing these elements, a fantasy author can use them to build an entire world.

For example, by using rotspren to push medicine forward, Sanderson neatly escapes a typical trap of medieval fantasy. By rights, any given scratch on a battlefield should have a significant risk of death. Alethi armies still lose as many men to rotspren as to enemy arrows, but there are battlefield medics with good antiseptics and moderately clean medical facilities with proper triage procedures in place to lighten the losses. A soldier with a medical background and training in field first aid can save many lives, and a soldier whose wounds are not too severe and who receives prompt medical attention has an odds-on chance to live. In other fantasy universes this problem has to be dealt with using magical healing. For example, consider Frodo Baggins, who was stabbed with a Morgul blade. Within days he was desperately ill, and without immediate attention from the most magical healers on the continent, he would’ve had no chance to survive. Without antispetics just about every wound works that way.

When there’s a reasonable chance to heal wounded soldiers, every loss can feel like a failure, and the obsession of protecting a squad can become all-consuming. Enter Kaladin Stormblessed, the main character of The Way of Kings. Kaladin is the son of a surgeon who left home and became a soldier, and he holds every death he can’t prevent as a personal sin. He spends the entire book striving to achieve a goal that may be impossible, but in most other fantasy worlds would be simply inconceivable.

And, of course, the existence of medicine enables many other plot developments. After all, how else would you justify a city like Kharbranth having so many hospitals? Best to say no more about that for now.

Roshar is a world with beautiful contradictions. In many ways, the spren do inhibit thought—after all, they do believe that things fall from the sky because groundspren are pulling on them—but they also cultivate curiosity and drive scientific endeavor. Sanderson shows us multiple spren researchers who have devoted their lives to the taxonomy and physics of these creatures. There’s even an elderly couple who seem to be drawing close to some kind of Heisenberg uncertainty principle by researching spren. Complicating this is the fact that we have to question our assumptions that we know more about how science should work on this world. I assume that rotspren are drawn to infection, rather than causing it, but it could easily be the other way around. Where does Roshar diverge from Earth? How far can we trust our scientific instincts? And if rotspren provide reliable information, how are we supposed to feel about creationspren and logicspren? Maybe we have to let magical spirits decide what is beautiful and logical. After all, they taught us how to do surgery right. I figure we owe them this.

Carl Engle-Laird is the Production Assistant for Tor.com. A spren told him to write this post, and that is totally healthy. You can find him on Twitter.