Oral thrush is a fungal infection that appears as a persistent, curdy white coating on the tongue, gums, or inner cheeks that does not come off with gentle cleaning. It is common for it to pass back and forth between a baby's mouth and the mother's nipples, so even if the mother has no symptoms, treatment for both is often necessary to prevent recurrence. Thrush can cause irritability and latching issues during feeds; consult your pediatrician for antifungal medication and ensure you practice thorough sterilization of bottles and pump parts.