This paper discusses the distinction between the transmission of infectious diseases within the domestic domain (the area normally occupied by and under the control of a household) and that in the public domain, which includes public places of work, schooling, commerce and recreation as well as the streets and fields. Whereas transmission in the public domain can allow a single case to cause a large epidemic, transmission in the domestic domain is less dramatic and often ignored, although it may account for a substantial number of cases. Statistical methods are available to estimate the relative importance of the two. To control transmission in the public domain, intervention by public authorities is likely to be required. Two examples show how environmental interventions for disease control tend to address transmission in one or the other domain; interventions are needed in both domains in order to interrupt transmission.