I have researched all 56 combinations of herbs for the Nature potions, and there are 3 of them that will actually make your villagers sick. (You get the message "That potion was awful!") You could always MAKE them sick, then have your doctor heal them.


SPOILERS

Combos to make your villagers sick -
Rose, Black Orchid, Lotus
Black Orchid, Cactus, Cactus
Tiger Lily, Pitcher Plant, Pitcher Plant

End Spoilers