Diaper rash: An inflammatory reaction localized to the area of skin usually covered by the diaper. It can have many causes, including infections (yeast, bacterial, or viral), friction irritation, chemical allergies (perfumes, soaps), sweat, decomposed urine, and plugged sweat glands. Most diaper rash problems can be solved by cleansing the skin with nonperfumed, gentle products; changing diapers frequently; and exposing the affected skin area to air. Commercially available diaper rash ointments may be helpful for prevention but may actually cause further irritation if used on the inflamed areas. Also called diaper dermatitis.

Diaper rash is characterized by the following:

 Skin signs. Diaper rash is marked by red, tender-looking skin in the diaper region — buttocks, thighs and genitals.
Changes in your baby's disposition. You may notice your baby seems more uncomfortable than usual, especially during diaper changes. A baby with a diaper rash often fusses or cries when the diaper area is washed or touched.

contact with urine and feces:
    Prolonged exposure to urine and feces can irritate the skin. Both urine and feces can cause moisture to come into contact with the skin, which makes it more prone to damage and irritation. Contact with digestive enzymes found in feces can also increase the risk of diaper rash. Although ammonia (the chemical that gives stale urine its smell) is irritating to the skin, the levels found in infant urine are not sufficient to cause a diaper rash. However, it can worsen skin that is already irritated and inflamed.
method of feeding:  
     Breast-fed babies may experience fewer diaper rashes than bottle-fed babies because breast-fed babies tend to have stools of a smaller volume, which in turn are less irritating to the skin.
friction and rubbing:
    Tight-fitting diapers that chafe against the skin can lead to a diaper rash. This damage to the skin can be made worse if the skin is wet. Also, skin-to-skin contact within skin folds in the diaper area can promote a diaper rash.
contact with irritating chemicals:
    Your baby's bottom is very delicate. Some common chemicals that are found in fabric softeners, detergents, baby lotions, fragrances, soaps, and baby wipes can be very irritating to the skin and should be avoided. Talk to your child's doctor or pharmacist about which products to avoid.
infections:
   The dark, damp, and moist environment created by a diaper is a perfect breeding ground for bacterial or yeast infections (e.g., Candida) on the skin. These types of infections are more common in babies who have a diaper rash. Blisters, pus, red bumps in the creases of the skin, "satellite" red areas outside the main redness, or severely swollen red areas may be signs of infection.

http://www.medbroadcast.com/condition/getcondition/diaper-rash
https://www.mayoclinic.org/diseases-conditions/diaper-rash/basics/symptoms/con-20019220
https://www.medicinenet.com/script/main/art.asp?articlekey=2982