This Element analyses how emotion was provoked by uncertainty in medical knowledge during the third plague pandemic in Sydney.

When the third global plague pandemic reached Sydney in 1900, theories regarding the ecology and biology of disease transmission were transforming. Changing understandings led to conflicts over the appropriate response. Medical and government authorities employed symbols like dirt to address gaps in knowledge. They used these symbols strategically to compel emotional responses and to advocate for specific political and social interventions, authorising institutional actions to shape social identity and the city in preparation for Australia's 1901 Federation. Through theoretical and historical analysis, this Element argues that disgust and aversion were effectively mobilised to legitimise these actions. As an intervention in contemporary debates about the impact of knowledge on emotion and affect, it presents a case for the plasticity of emotions like disgust, and for how both emotion and affect can change with new medical information.