coccus aureus may interfere with the subsequent acquisition of another staphylococcal strain; (2) this phenomenon can be utilized to protect an infant against infection and disease caused by the so-called epidemic phage types; and (3) certain staphylococcal types are more pathogenic to infants than others.1-4These studies were carried out in hospitals which cared for different population groups and engaged in a variety of nursery practices. The epidemiology of infectious disease in general is complex and perhaps even undefinable when several mutually independent factors operate simultaneously.