Cordyceps is a genus of ascomycete fungi (sac fungi) that includes about 400 identified species and many yet to be described. All Cordyceps species are endoparasitoids, mainly on insects and other arthropods (they are thus entomopathogenic fungi); a few are parasitic on other fungi. The best known species of the genus is Cordyceps sinensis,^[1] first recorded as yartsa gunbu in Tibet in the 15th Century. It is known as yarsha gumba in Nepal. The Latin etymology describes cord as club, ceps as head, and sinensis as Chinese. Cordyceps sinensis, known in English commonly as caterpillar fungus, is considered a medicinal mushroom in oriental medicines, such as traditional Chinese medicines and traditional Tibetan medicine.

When a Cordyceps fungus attacks a host, the mycelium invades and eventually replaces the host tissue, while the elongated fruiting body (ascocarp) may be cylindrical, branched, or of complex shape. The ascocarp bears many small, flask-shaped perithecia containing asci. These in turn contain thread-like ascospores, which usually break into fragments and are presumably infective.

There are over 680 documented species of the sac fungus genus Ophiocordyceps, and one of the best known of these is Ophiocordyceps sinensis, colloquially known as caterpillar fungus. The fungus is known in Tibetan as yartsa gunbu or yatsa gunbu.

Caterpillar fungi are the result of a parasitic relationship between the fungus and the larva of the ghost moth genus Thitarodes, several species of which live on the Tibetan Plateau (Tibet, Qinghai, West-Sichuan, SW-Gansu & NW Yunnan), and the Himalayas India, Nepal, Bhutan). The fungus germinates in living organisms (in some cases larvae), kills and mummifies the insect, and then the fungus grows from the body of the insect.

O. sinensis is known in the West as a medicinal mushroom, and its use has a long history in Traditional Chinese medicine as well as Traditional Tibetan medicine.^[2] The hand-collected fungus-caterpillar combination is valued by herbalists and as a status symbol;^[3] it is used as an aphrodisiac and treatment for ailments such as fatigue and cancer, although such use is mainly based on traditional Chinese medicine and anecdote.