Thrush in a baby's mouth is a yeast infection. A yeast infection is a fungal infection, but not all fungi are yeasts.

A yeast called Candida albicans causes thrush. There are many candida species, but albicans causes most human infections and is almost always the cause of thrush.

Thrush infects newborn babies up to three to four weeks old. As a baby grows, he or she will no longer be susceptible to thrush. Healthy adults and children with normal immune systems should not get thrush.

A baby's mouth...