Tinea cruris, commonly referred to as jock itch, is a fungal infection in the groin area. It affects mostly adult men. It's caused by a type of fungus called dermatophytes. It grows in a circle on your skin and often looks like a ring. Due to irritation, the skin becomes red.
Some people think the infection looks like there is a worm under the skin. Because of this, this infection is often referred to as "ringworm," although no worm is actually involved. The fungi are commonly found on the skin surface and nails and are usually quite harmless. Under certain conditions, however, they can multiply dramatically, causing irritation and surface tissue damage.
You don't have to be athletic to get jock itch. Infections caused by these organisms go by the medical name tinea. Tinea pedis is athlete's foot; tinea capitis is ringworm of the scalp. Jock itch is tinea cruris, from the Latin word for leg. They are all caused by dermatophytes - only the infected area differs.
Classic jock itch affects the inner thighs, groin, and lower buttocks, not the genitalia themselves. If the scrotum is inflamed, it's probably due to a different yeast condition called candidiasis, and a physician should be consulted.