Rash on an infant's buttocks caused by Candidiasis, an infection with a yeast-like fungus of the genus Candida (usually Candida albicans). The usually superficial infection can occur in many areas of the body, mainly in the moist body folds, the vagina & the respiratory tract (candidiasis of the mouth & vagina is known as thrush). The infection may present itself in persons taking broad-spectrum antibiotics; it is treated with another antibiotic, nystatin.