Many long-distance runners and body builders take ginseng to heighten physical endurance and improve stamina. Some nutritionally-oriented doctors and herbalists believe that ginseng is able to delay fatigue because it enables the exercising muscles to use energy more efficiently. There is research, however, that contradicts this hypothesis. Ginseng may help the body combat a variety of illnesses. It stimulates the immune system in a number of ways, including production of specialized immune cells called killer T cells, which destroy harmful viruses and bacteria.