For all of its efforts to remain realistic, Kingdom Come: Deliverance is still a video game, and needs to pull some video game-y moves to keep players entertained. This means a variety of sidequests much wider than would realistically fit into the skillset and ability of a blacksmith apprentice-turned-adventurer - and this includes curing a sickness.
That’s right, the Pestilence side-quest has Henry create a cure for a small-scale outbreak of some plague or other, and it rewards you with an achievement for your troubles as well. You’re not likely to miss this quest, as it intersects with some frequency with one of the game’s main quests as well.
Pestilence involves a lot of running around playing errand boy, with most objectives consisting of speaking to people. While not a prerequisite of this quest, having completed Mightier than the Sword and thus becoming literate will yield a better outcome than going into it without being able to read. The first half of the quest is pretty straightforward. You need to head to Merhojed, speak to Mattias, then Melichar, then Bedrishka. Here, you take a small breather from all the yapping with the nary too cheerful task of digging a grave for her husband and burying him.
Then you’re off to speak with Daniel, then Vincent then Old Straw. At this point it’s clear that something must be done about the sickness that’s running rampant, so you head to Sasau Monastery and speak to Johanka. After this, you speak to Brother Nicodemus, but before you talk about the plague itself, make sure to purchase two Thistles and Valerian from him, then field the topic of the plague. As the conversation proceeds, inform the Brother that half the houses and all the animals have fallen ill, then tell him you can read.
If you haven’t completed Mightier than the Sword, you’ll have to describe the symptoms of the disease, leaving the rest of the cure-brewing to Nicodemus, resulting in some of the sick villagers dying before you can save them. Once you’ve told Nicodemus that you can read, head on over to the lectern and go through the book. Henry, now armed with complete pathological knowledge and having become a veritable walking medical lexicon, tells Nicodemus that he knows what ails them [the villagers].
You need to say that poisoned water is the cause of the sickness, and that you can help the village with a cure. There is an option for Nicodemus to brew the potion, just like if you can’t read, but again, this yields an unfavorable result and no achievement. With the Valerian and Thistle already in your possession, you need only charcoal, which is easy to acquire from the blacksmith in Samopesh.
Once you have the ingredients, head to an alchemy station. The recipe for Aid for Merhojed isn’t too complicated, but alchemy can be a tad fiddly in this game even if you know what the sequence of ingredients is. First, you need to get the water boiling, add the two Thistles, boil for two turns, add the Valerian, boil for one turn, add the charcoal after the flames went out then decant in a phial.
After this bit, you’re back to running errands, basically. Hop back to Brother Nicodemus, then off you go to give the cure to Straw, Vincent, Daniel and Bedrishka. After this, you’ll need to speak to Melichar about the captured bandit who is also sick. Melichar will let you see the bandit whom you need to cure and then interrogate. After this, in a scripted event the villagers will try to mob the bandit, but you need to persuade them to stand down, and then take the bandit to the Rattay Prison.
While the quest is technically done, you need to wait a few in-game days then go back to Merhojed and speak to Melichar again about how the situation in the village is. At this point, you’ll receive your Plague Doctor achievement, and the quest will truly be done - congratulations! Henry is now a medical professional. If only it were so easy these days.
