Newborns' immune... ...Systems are 'weaker' (less experienced) than those of older children. Candida, the fungus that causes thrush, lives in most peoples' mouths and intestines without causing any trouble, but can overgrow and cause thrush in small children. It is fairly easy to treat, and practically all children grow out of having them.
Yeast infection. Thrush is caused by yeast. Yeast are everywhere. In young infants, who have less "normal flora" than older children (normal flora refers to the normal and healthy germs we carry in our mouths), yeast sometimes gets out of control, causing white patches in the mouth. Thrush is also more likely when the baby is on antibiotics which also decrease normal flora.