Thrush is an infection caused by Candida. It is very common.
You will be able to see white spots, plaques, or a thick white coating in the baby’s mouth (often in a baby who doesn’t want to feed but sometimes in an asymptomatic baby), and/or a refractory nappy rash.
In breastfeeding, it can be passed between the mum and the baby and can make breastfeeding painful.
In some babies, no treatment is needed. Many mild cases of oral thrush only last for just a few days and self-resolve. Treatment involves an antifungal treatment (e.g. oral nystatin or Daktarin gel). In breastfeeding babies, the mum should also have treated for Candida.