Skin wetness is the common denominator underlying the various causes of diaper rash. Urinary wetness increases skin friction, raises the skin pH, makes the skin less cohesive, and makes it more permeable. These effects combine to intensify the action of stool enzymes or other irritants that then inflame the skin. With this protective layer breached, it is easy for microorganisms such as yeast or bacteria to invade the inflamed skin. This makes the rash worse and less responsive to usual treatments.
Yeast is by far the most common type of organism found in a diaper rash.
The organism is quite prevalent and thrives in warm, moist skin. Yeast involvement should be suspected in any diaper rash that has not improved dramatically with 72 hours of appropriate therapy. Current or recent antibiotic use makes a yeast infection even more likely, since this reduces the amount of the skin's 'good' bacteria that fight infection. Classically a yeast rash is beefy red with sharp raised borders and white scales. Small satellite lesions surround the main rash.