Thrush (candidiasis) is a fungal infection in the mouth. It is caused by yeasts belonging to the genus Candida. An overgrowth of yeast causes white patches on the corners of the mouth, the insides of the cheeks, the tongue, roof of the mouth and throat. It is common in babies and can be acquired during delivery. Older children, adults, or the elderly are more likely to get thrush if they are diabetic, malnourished, wearing dentures, or receiving treatment with steroids, antibiotics, or cancer chemotherapy