Doctors have long known that infants who are breast-fed contract fewer infections than do those who are given formula. Until fairly recently, most physicians presumed that breast-fed children fared better simply because milk supplied directly from the breast is free of bacteria. Formula, which must often be mixed with water and placed in bottles, can become contaminated easily. Yet even infants who receive sterilized formula suffer from more meningitis and infection of the gut, ear, respiratory tract and urinary tract than do breast-fed youngsters.