Goldenseal (Hydrastis canadensis) grows in eastern North America, where it's now threatened in the wild. An alkaloid in goldenseal, berberine, shows powerful antimicrobial effects against a wide range of bacteria, yeasts, and parasites. Herbalists prescribed goldenseal to stimulate the immune system, fight infection, and treat diarrhea.
(Podcast: The Plant Detective, 9/13/14)