Dental caries in babies and toddlers is called Early Childhood Caries (ECC). It is an infectious and transmissible die-to-bacterial disease. Detailed knowledge regarding the acquisition and transmission of infectious agents facilitates a more comprehensive approach toward prevention. Mutans streptococci are important organisms in the initiation and progression of dental caries. Recent evidence demonstrates that these bacteria are found in the mouths of pre-dentate infants and are acquired via vertical and/or horizontal transmission from human reservoirs. This information should facilitate the focusing of clinical interventions that prevent or delay infant infection, thereby reducing the prevalence of dental caries (ECC) in babies and toddlers.