Cordyceps is a large fungal genus with an estimated 400 species. This genus is of note because of the way in which it reproduces: the fungi parasitize insects or plants, and then produce a fruiting body which sprouts directly from the body of the host. As if this wasn't enough, some species which attack insects are actually capable of controlling their hosts and forcing them to engage in activities which will further the cause of the fungus. In other words, some species of Cordyceps are capable of mind control!
On species in this genus, C. sinensis, has been used in traditional medicine in Asia for centuries. This particular representative of the Cordyceps genus attacks caterpillars while they burrow underground, encouraging the caterpillars to come to rest near the surface so that the fungus can put out a fruiting body which will emerge above the soil. The roughly club-shaped fungus comes from the caterpillar's head in most cases.
The resulting caterpillar-fungus combination is known variously as semitake, Chinese caterpillar fungus, vegetable caterpillar, hsia ts'ao tung ch'ung, dong chong xia cao, and yarsha gumba. It is supposed to confer the benefits of vitality and energy, and is used to treat several conditions. Most notably, the fungus is often prescribed to people with sexual dysfunction, especially men, but it is also believed to benefit the immune system, liver, kidneys, and heart, and it is used to treat bronchial conditions as well.
Analysis of C. sinensis suggests that the fungus contains compounds which stimulate the T-cells in the immune system, which means that some of the uses of this fungus are not entirely far-fetched. The stimulation of helper cells does indeed boost the immune system, and some of the compounds in the fungus appear to promote better liver function. While the fungus does not act as an aphrodisiac, it also contains chemicals which would address erectile dysfunction, as well.
Cordyceps fungi are available from some Asian markets and health food stores. Consumers should be careful about purchasing this and other substances used in Traditional Chinese Medicine because regulation and enforcement of safety is erratic. Many dried fungi can look like Cordyceps, making it difficult to confirm that one is looking at the right fungus, and while the fungus itself does not appear to be toxic, samples contaminated with lead have been found in numerous markets, which is an issue of concern. Going through a reputable Chinese herbalist is strongly recommended, as is being prepared to pay a fairly high price.