Cordyceps, a fungus that grows on caterpillars high in the Himalayas, has long been used in Traditional Chinese Medicine as a rejuvenating and performance-enhancing medicine. The latest Korean research shows it also boosts the immune system. Scientists tested 79 healthy adults for eight weeks, giving 39 of them 1,680 milligrams of cordyceps a day in capsules and the other 40 were given a placebo. The cordyceps produced a 38 percent increase in natural killer (NK) cell activity, which plays a role in immunity by detecting and killing virus-infected cells, tumor cells and abnormal cells.