Brown ring disease (BRD) in the Manila clam is induced by the bacterium Vibrio tapetis. During the infection process, the pathogen enters the extrapallial compartment of the Manila clam and induces the formation of a characteristic brown deposit that gives the disease its name. Although post-infection processes have been widely described for this disease, the mechanisms of entry of the bacteria into the extrapallial compartment remains unclear. From relationships between clam size and BRD prevalence, and between grain-size distribution in natural habitats and prevalence, we propose a simple explanation for this step: V. tapetis benefits from mechanical disruptions of the periostracal lamina or valve margins to colonize the extrapallial compartment. Such disruptions may be induced by the presence of large sediment grains in natural habitats, which become lodged in the shell opening. This hypothesis suggests that limiting handling of clams may help to limit development of BRD in cultured clam beds.