Respiratory and gastrointestinal infections are common in children under the age of 4 years, especially after the start of schooling. These conditions are facilitated by a still incomplete functional maturation of the immune system and the anatomical structure and function of the respiratory and gastrointestinal tract still developing. The frequency and duration of these conditions involves a high discomfort and significant costs, in relation to medical appointments, taking medication, the need for hospitalization, days of absence from school and work days lost by parents. Functional foods derived from the fermentation of cow's milk with probiotic strains have been proposed for the prevention of infectious diseases in children. Several products have been investigated, with sometimes conflicting results. Diversity in experimental designs, populations evaluated, and bacterial strains used in the preparation of fermented products are probably responsible for these discrepancies. Recently we started a study approved by the Ethics Committee for Biomedical Activities "Carlo Romano" of the University of Naples "Federico II" (protocol number 210/12) to evaluate the effectiveness of foods fermented with Lactobacillus paracasei CBA-L74 in the prevention of common winter infections in school children aged between 12 and 48 months. Studies of pre-clinical phase showed anti-inflammatory activity of milk fermented with the strain Lactobacillus paracasei L74-CBA in terms of stimulation of the production of the cytokine IL-10 and decreased synthesis of IL-12, also in response to stimulation with Salmonella typhimurium. The data were obtained in in vitro studies on dendritic cells and ex vivo intestinal biopsies as well as in tests on healthy mice and on a mouse model of experimental colitis. A preliminary analysis of the data was found that subjects treated with fermented milk showed fewer infectious episodes, as well as a lower incidence of respiratory tract infections or gastrointestinal, with a statistically significant difference between the study groups. It was also observed a significant increase in the levels of α- and β- defensins, LL-37 and secretory IgA in the group of subjects treated with fermented milk compared to subjects treated with fermented rice or placebo. Therefore, we decided to extend the period of study of five additional months, in order to perform an evaluation of the effectiveness of fermented milk (which was more effective)vs placebo.