The results of a study of the effect of probiotic containing Lactobacillus fermentum CECT5716, isolated from breast milk and used for the prevention of postpartum mastitis, on the health of infants in the first half of life, in particular, the incidence of respiratory and gastrointestinal tract infections, are presented. It was found that infants of the main group (234 patients), whose mothers received a probiotic for a month, based on lactobacilli Lc40, 3.2 times less likely to have acute respiratory infections and 8.0 times less often from diseases of the gastrointestinal tract of various origins, compared with children (39 infants) fed unfortified milk (р < 0.05).