Certain germs normally live in our bodies. While most germs are harmless, some can cause infection.
Thrush occurs when too much of a yeast called Candida albicans grows in a baby's mouth. Germs called bacteria and fungi naturally grow in our bodies. Our immune system helps keep these germs in check. But babies don't have fully formed immune systems. That makes it easier for too much yeast (a type of fungus) to grow.
Thrush often occurs when mother or baby has taken antibiotics. Antibiotics treat infections from bacteria. They can also kill "good" bacteria, and this allows yeast to grow.
The yeast thrives in warm, moist areas. The baby's mouth and the mother's nipples are perfect places for a yeast infection.
Babies can also get a yeast infection on the diaper area at the same time. The yeast gets in the baby's stool and can cause a diaper rash.