There are several herbs that act as immuno-stimulants, botanical medicines that help your body fight off illness by bolstering its natural defenses. Echinacea is one of the most popular medicinal herbs used in North America, and for a good reason as it has the most scientific evidence behind it. This herb kicks your immune system into high gear. Echinacea may help stop a cold, influenza, or bacterial infection before it can spread in the body. It may also shorten the duration and lessen the symptoms of infection, according to some doctors.