If someone is sick feed them charcoal to heal them. Okay, so maybe you shouldn't feed it directly to them, but charcoal has amazing water filtration powers. So put charcoal in their water and the water will heal them right up.
You aren't the only one that needs to be healed, so you take your row (りょう) boat out onto the lake where your town gets its drinking water and you dump a bunch of charcoal into it! Now everyone will be healed when they drink.