You'll know your baby has diaper rash if you notice that the skin on the bottom is irritated, red, and warm to the touch. The most likely culprit of irritation like this is wetness from your baby's own urine. When urine mixes with bacteria from your baby's poop, the result is ammonia, a harsh chemical that can further aggravate skin. Babies with sensitive skin may also experience diaper rash as a result of sensitivity to fragrances in a disposable diaper, or to chemicals in skin care products. Additionally, when your baby is exposed to new foods, the composition of his or her poop may change, in turn, resulting in a rash. Finally, diaper rash can be caused by a yeast infection, which occurs when the yeast fungus grows in the diaper region, nurtured by the warm and wet environment of your baby's diaper. Unlike standard issue diaper rash, this angry, red infection will have slightly raised borders, and may spread as lesions on nearby body parts.