Diaper rash may be more common than you think. More than half of babies 4 to 15 months old get diaper rash at least once in a two-month span, say experts at the American Academy of Pediatrics (AAP).

"Most cases are not serious, but diaper rash requires attention by the parents or caregivers, and it can become a serious problem if not treated at home or by a physician," says Winston Price, M.D., of the AAP's Practice and Ambulatory Medicine committee.

The top cause of diaper rash is moisture, made worse by chafing or rubbing. Moisture often stems from long contact with urine or feces. "It's important to check the baby's diaper often and change it," Dr. Price says. Disposable diapers are better than cloth diapers at absorbing urine and keeping the baby's skin dry.

A simple case of diaper rash should clear up in three to five days, Dr. Price says. If you let moisture and skin irritation continue untreated, your baby can come down with a yeast or bacterial infection resulting in raised skin bumps that look like pimples and possibly blisters or crusting.

Candidiasis, an infection by a yeast called Candida albicans, is one of the most common infections. The pediatrician usually treats it with nystatin cream, although other antifungal medications such as clotrimazole also will work. If the infant has candidiasis in the mouth, a condition called thrush, the doctor usually will prescribe oral nystatin. Streptococcal and Staphylococcal skin infections also occur in the diaper area. These infections often are called impetigo, or, if a blister is present, bullous impetigo. These are treated with either oral or oral and topical antibiotics.

"The key is not to wait," warns Dr. Price. "If a simple case of diaper rash does not resolve in a week or two, or it worsens sooner, call your pediatrician."

Prevention is the key, says Dr. Price. "Change the diaper frequently, dry the baby thoroughly and use good, routine skin care." That includes gentle cleansers and moisturizers to create a barrier between the skin and germs growing in excess moisture.