It is very common to see thrush in infants under a year of age. Infants are first exposed to yeast before they are born, or after they pass through the birth canal, as most women have some yeast in the vagina or on their skin. For some infants, the yeast grows quickly in the mouth. It is more likely to grow if the child has been on antibiotics recently, but there usually isn't a specific stimulus. Thrush does not generally cause many symptoms, though some babies may have some discomfort while they are eating or sucking on a bottle. In normal healthy children, the yeast does not really cause an "infection," but rather just sits there in the mouth. It is not common to see thrush in people over about a year of age, as the body's immune system has matured by then, and will prevent it from overgrowing. In children who have an abnormal immune system, yeast can cause more serious illness and infection.
The usual treatment for thrush is to have the baby take a prescription medicine called nystatin. The nystatin is put in the mouth with a dropper so that it runs along the inside of the cheeks, and then it is swallowed. It is not absorbed into the bloodstream, unlike most other medicines that are taken by mouth, and it comes out in the stools. It works by having direct contact with the yeast in the mouth. For most babies, this treatment will cure the yeast overgrowth, though it may take a while, and thrush can sometimes be difficult to get rid of. Some mothers can actually have the yeast on their nipples, and so the baby is re-exposed to it frequently. If this is the case, the nystatin is also available in a cream that can be rubbed on mom's breast. Yeast can also sit on the plastic nipples for baby botttles, though it is easy to get rid of just by boiling the nipples for a few minutes. For thrush that is persistent despite these measures, there are other medications available such as ketoconazole, and you should talk with your physician about whether it would be reasonable to use them.