‘Risk’ has long been at the centre of expert and popular perceptions of transmissible and stigmatised blood-borne viral infections, such as HIV and viral hepatitis. There is a substantial body of research on transmission risk among couples with mixed viral infection status (serodiscordance). But we know very little about how families affected by HIV and viral hepatitis engage with understandings of infectiousness and how these shape family relationships in different ways. Guided by cultural theories of risk that build on Mary Douglas’ work, we draw on qualitative interviews to explore the ‘performativity’ of risk in serodiscordant families in Australia. We show how the ‘doing’ of risk could be constitutive of difference, which unsettled the family connection or deepened existing fault lines. Conversely, the ‘undoing’ of risk enabled the preservation of the family bond by rejecting difference and reframing risk as an external threat to the family in the form of stigma. We conclude that risk in the context of serodiscordant families had relational implications far beyond viral transmission and consider what our findings might mean for service provision and health promotion campaigns related to blood-borne viruses.