According to NHS, Oral thrush is caused by a yeast fungus called Candida albicans. Oral thrush in babies isn't usually serious. Healthy people have this fungus in their mouths and it doesn't normally cause problems. This can also make you prone to a thrush infection that may then be passed to your baby during breastfeeding. You should get advice from a pharmacist if you think your child may have the condition. Treatment with a mouth gel usually lasts at least 7 days. Always follow the instructions on the medicine packet.