There may be times when your baby has a certain rash on some portion or all portions of their body. Rashes can be caused by a host of reasons. Sometimes viral reactions can cause a rash throughout the body. Sometimes a rash is present with a fever. If a rash is present with a fever it's usually the sign of something more than just a topical reaction. You would want to bring the baby in to a physician to be seen.
If it's just a rash on the skin and nothing else is wrong with the baby, there's just some sort of topical irritation going on with the skin. This could be caused by a new laundry detergent or a piece of clothing that may be irritating their skin. Babies can react poorly to certain soaps that may be stronger or more irritating. Your baby could have eczema and could be better with a thicker, more treatable eczema cream.
There are also something called yeast infections which both boys and girls can get in the diaper region and all over their body. And this is little tiny raised bumps. This sometimes present itself more often in the summer months because in hot moist areas yeast tends to grow.
So anytime you notice a rash that looks different and you don't think it's from chafing of clothing or new laundry detergent, it's really best that you go to your pediatrician or your doctor to help you figure out what that rash is and figure out the best course of treatment, because there are some prescriptions that are needed to help treat a variety of infant rashes.