If you can arrange it, allow the baby to go without diapers for at least part of the day to let the air reach the sore skin. Once the diarrhea gets better, the rash will usually go away within a few days. Sometimes babies get a yeast infection in the diaper area. With yeast, you will usually see some large areas of bright redness, with smaller red dots around them. These rashes often will not get better with just with good hygiene and exposure, but require treatment with a prescription cream.
Yeast infections are more common in children who are taking antibiotics.
If you suspect your child has a yeast infection, call your pediatrician. Babies in the first few weeks of life often get some irritation in the diaper area that is not serious. But we do get concerned about infants who have large blisters in that area. If this happens, call your pediatrician. This may be an infection that needs treatment.