Thrush is a yeast. It lives everywhere on our bodies and in the environment. But it only 'overgrows" when the immune system is challenged and can't deal with it. (so you are more likely to get thrush for instance, when you are pregnant) Babies are more likely to get thrush from bottle feeding equipment where a moist warm environment allows it to overgrow. It looks like a whitish, cottage cheesy substance on the inside of baby's lips and cheeks. It is unlikely that you would get it on your nipples and the theory of yeast in the ducts of the breast is almost entirely discredited. Often thrush is misdiagnosed when the infection is a bacterial one. but what your baby has sounds like just a normal whitish tongue.