The initial stages of a disease outbreak can determine the magnitude of an epidemic. Though rarely tested in unison, two factors with important consequences for the infectious transmission dynamics are the traits of the susceptible population and the traits of the index case (i.e., “patient zero”). Here, we test whether the behavioural composition of a social group can explain horizontal transmission dynamics of cuticular bacteria using a social spider. We exposed focal spiders of known behavioural types with a GFP-transformed cuticular bacterium (Pantoea sp.) and placed them in groups of 10 susceptible individuals. We measured bacterial transmission to susceptible spiders composed of either all shy spiders, 10% bold spiders, or 40% bold spiders. We found that colonies with 40% bold spiders experienced over twice the incidence of transmission compared to colonies with just 10% bold individuals after only 24hr of interaction. Colonies of all shy spiders experienced an intermediate degree of transmission. Interestingly, we did not detect an effect of the traits of the index case on transmission. These data suggest that the phenotypic composition of the susceptible population can have a greater influence on the degree of early transmission events than the traits of the index case.