Improper laundering of diapers. Family history of skin allergies. Hot, humid weather. For cloth diaper, rinse them twice to remove detergents and other chemicals after washing. Leave diaper off for 10-30 minutes between diaper changes for air exposure.
Corn starch reduces friction and can be used at diaper changes to prevent diaper rash.
Avoid talcum powder because of the risk of inhalation pneumonia. Change diapers frequently. Avoid rubber or plastic pants. Cleanse with cotton dipped in mineral oil, do not use soap after every diaper change. Use a mild soap such as Dove soap after bowel movements, and rinse thoroughly.