From high elevations of the Tibetan plateau comes a minute fungus known as Cordyceps sinensis. It parasitizes on caterpillars and sprouts out of their dead carcasses.  With 95% of the world’s mushroom species still undiscovered, it’s a small wonder that a tiny fungus from a remote and sparsely populated area is not only described but revered the world over as one of the most highly prized medicinal mushrooms on the planet.
The unlikely origins of Cordyceps sinensis as a medicinal mushroom is enough to stand up and take notice.
Cordyceps extract has been valued in China for almost two thousand years as an aphrodisiac and general vitalizing tonic. The first recorded mention of Cordyceps extract appears in the Chinese work The Classic Herbal of the Divine Plowman from about 200 A.D.  Modern works have reaffirmed its traditional wisdom. 
Contemporary researchers in Asia have discovered that Cordyceps sinensis demonstrates anti-tumor and immune-enhancing qualities. [4, 5, 6, 7, 8, 9, 10, 11, 12]
In particular, a number of studies indicate its potential usefulness in cases of leukemia. [13, 14, 15, 16] In 2002, the U.S. National Institute of Health began Phase I screening of cordycepin in the treatment of leukemia. Cordycepin is an active compound from Cordyceps sinensis.
One clinical study on Cordyceps extract showed a 64% improvement in the sexual performance of men with sexual dysfunction.  The underlying reason for Cordyceps effect as an aphrodisiac may be that it appears to help increase blood-flow by dilating blood vessels. [18, 19]
Cordyceps extract also appears promising for cardio-vascular health by reducing cholesterol and acting as a general cardiotonic,  as well as aiding the kidneys  and liver (post-hepatitic cirrhosis). 
However, Cordyceps’ popularity is primarily based on its effect on physical empowerment by helping to increase stamina and endurance. [23, 24, 25] Part of the explanation to this may be, as mentioned earlier, that it has an effect on improving blood-flow by dilating blood vessels. Another reason may be the similar effect it has on also helping to dilate respiratory passageways, which was demonstrated by two studies on bronchitis and asthma. [3, 26]
Nine Chinese women who set world records at the 1993 Chinese National Games attributed their success to their use of Cordyceps extract. [27, 28]
Cordyceps extract is a confirmed source of anti-oxidants,  polysaccharides, and a large host of other newly discovered nutritional compounds, [8, 30, 31] plus certainly many others that are yet to be discovered.
In his book Mycelium Running, medicinal mushroom expert Paul Stamets also lists the following areas where research has shown Cordyceps extract to have a beneficial effect: Bacteria; viruses; blood pressure; nerve tonic (stress); lung cancer; lymphoma. 
Note: The statements on this page have not been evaluated by the Food and Drug Administration. This article is not intended to diagnose, treat, cure or prevent any disease. Always consult a licensed medical practitioner before using any herb (or mushroom) for medicinal purposes.
Credit: Thank you to Paul Stamets for source material.