Simple knowledge, such as the disease model of sickness and "microorganisms exist, keep things clean" would be useful. You could probably cobble together a water-drop microscope and demonstrate the difference between (say) clean wounds and infected ones. Fleas = plague. All that.
Knowing how to distill hard liquor would make you popular as well.
Making hard liquor worth drinking is surprisingly difficult. It's lot easier if you know how to make activated carbon, but making activated carbon is difficult with equipment of that age(possible though).
Knowing how to distill hard liquor would make you popular as well.