The 1830s were dominated by the cholera pandemic (1826−37) and epidemics of influenza, typhus, and typhoid (1836−42). These events were so important at the time that the discourse of popular protest became interwoven with the language of contagion and of sanitary reform. The reformist unrest of the 1830s was recast in Charles Dickens’s Barnaby Rudge (1841) as the 1780 Gordon riots. This chapter explores the extent to which the political and religious unrest in Barnaby Rudge mimics epidemic transmission by placing the novel alongside modern epidemiological studies of urban riots. Further, Dickens connects the 1830s discourses of epidemic and riot with madness, focussing on the problem of the undiagnosability of madness. Barnaby Rudge raises important questions about the transmission of dangerous ideas. Moreover, it connects these to the problem of individual culpability in the case of intellectual disability.