Tinea cruris is the scientific name for fungal infection of the groin region. Official alternative names include Jock itch or dhobi itch (after Indian washermen).
Opportunistic infections (infections that are caused by a diminished immune system) are frequent. Fungus from other parts of the body (commonly tinea pedis or 'athlete's foot') can contribute to jock itch. A warm, damp environment allowing the fungus to cultivate greatly contributes; especially with tight, sweaty or rubbing clothing such as jockstrap or tight undershorts.
The type of fungus that most commonly causes tinea cruris is called Trichophyton rubrum. Some other contributing fungi are Candida albicans, Trichophyton mentagrophytes and Epidermophyton floccosum. [1]
1. http://en.wikipedia.org/wiki/Jock_itch