This is some info from patient UK

What is oral thrush?

Thrush is an infection caused by a yeast germ called Candida. The mouth is a common site where Candida causes infection. This is called oral thrush. Other common sites for thrush to develop are the vagina, nappy area, and nail folds.

Why do some babies get oral thrush?

Small numbers of Candida commonly live on healthy skin and in a healthy mouth. They are usually harmless. However, for reasons that are not clear, an 'overgrowth' of Candida occurs in the mouth of some babies. This can cause a bout of oral thrush. About 1 in 7 babies develop a bout of oral thrush at some point. It is most common in babies younger than 10 weeks, but it can occur in some older babies too. Oral thrush is not usually due to poor hygiene, and it does not usually mean that the baby is ill in any other way. Some babies have recurring bouts.

What are the symptoms of oral thrush?

White spots usually develop in the mouth and on the tongue. The spots may join together to form larger spots or 'plaques'. They may become yellow or grey.
The baby may not be bothered by the infection. However, sometimes the mouth becomes sore. Some babies may drool saliva, or refuse to feed properly because of soreness.
What is the treatment for oral thrush?

Many mild cases last a short time - just a few days or so - and clear without any treatment.
Your doctor may prescribe a gel that contains an anti-thrush drug called miconazole. You smear this onto affected areas in the mouth, as often as prescribed by your doctor. To avoid the very small risk of choking, apply a little at a time and not to the back of the throat. Little of this drug is absorbed into the body when swallowed. It works by killing the Candida germs within the inside of the mouth.
An alternative treatment is drops which contain an anti-thrush drug called nystatin. You use a dropper which comes with the drops to place the liquid onto affected areas.
See your doctor if the thrush has not cleared within seven days of starting treatment.
Can oral thrush be prevented?

Most bouts of oral thrush occur for no apparent reason. However, the following tips may help to prevent some bouts.

Regularly sterilise all dummies and other mouth toys used by a baby.
If you bottle feed, regularly sterilise all feeding equipment, especially teats.
Some people suggest giving a baby a drink of sterilised water after a feed. This rinses away any residual milk in the mouth which Candida may thrive on.