An overgrowth of the Candida fungus in the mouth or throat is what causes thrush. Thrush in infants is relatively common and is only a concern if it causes poor feeding or is associated with weight loss or other symptoms of systemic illness.n overgrowth of the Candida fungus in the mouth or throat is what causes thrush. Thrush in infants is relatively common and is only a concern if it causes poor feeding or is associated with weight loss or other symptoms of systemic illness.