I found out the hard way with Roxy. She began having dry flakey skin, then tremendous scratching all the time. I went through a few shampoos and settled on a Medicated Ones for skin issues. Well...totaly made her worse and she started to develop staph infections. This has been going on for almost 1 year. After doing some research, I do believe it is the following ingredient is the cause. Please check your labels, this stuff is in everything, if your baby has sensitive skin or skin issues, please research and find a product that is free of SLS:
Sodium Laureth Sulfate
Products containing these substances can affect those prone to eczema and other irritants. These substances provide a foaming quality to the product, allowing for better distribution of the product while washing hair or skin and while brushing teeth. When rinsed off, the product will have cleaned the area but will have taken moisture from the top layers of skin. In people with sensitive skin (prone to dermatitis, acne, eczema, psoriasis and chemical sensitivity), the drying property of these type of detergents can cause flare-ups of skin conditions or may worsen existing conditions.
http://en.wikipedia.org/wiki/Sodium_laureth_sulfate