Fungal infection in the skin of the genitals, inner thighs and buttocks. Jock itch is a fungal infection. Jock itch (tinea cruris) is a fungal infection that causes a red and itchy rash in warm and moist areas of the body. Jock itch is an infection called tinea cruris, and it's caused by a fungus. Jock itch is a common, itchy rash of the groin. Jock itch is a pretty common fungal infection of the groin and upper thighs.