There can be 4 types of rashes generally happen between 0-15 months of age:
Due to moisture and irritation
Due to yeast ie fungus
Due to bacteria
Due to allergy
First one due to moisture and irritation is most common . jab bhi hum baby ko god lete hai and also babies keep moving their legs constantly which causes the wet diaper to rub against their sensitive skin softened by moisture and poop
Second one is Yeast/Fungal rash which happens between 9-12 months of age when babies are on solid food. Yeast is naturally found in intestine and baby ke poop se spread hoke upper thighs, buttocks ya groin mein overgrown ho jata hai hai jahan 2 skin closed touch mein rehti hain aur clean nai ho pati hai
Third is Bacterial diaper rash which is rare. These are fluid filled blisters
Fourth is Allergic rash: It happens if your babies skin is sensitive to certain type of chemical. aapke lotion , powder, wipe ,detergent mein kuch aapke baby ke skin sensitive chemical ho sakta hai...eg phenoxythenol