This dissertation examines the early twentieth-century anxiety that disproportionately high rates of suicide indicated a suicide epidemic. A sense of the suicidal impulse as contagious and most likely to spread amidst the crowded urban environment is especially prominent in the period's scientific discourses, and this anxiety over public hygiene and population control emerges in a strand of modernist fiction that repeatedly portrays the suicidal subject as suffering from an intersubjective contagion rather than intrasubjective anomie. Thus challenging accepted critical narratives of urban suicide as the result of psychic isolation, texts by John Dos Passos, Virginia Woolf, Elizabeth Bowen, and Djuna Barnes suggest the necessity for a more epidemiological reading of self-destruction in modernist literature, and particularly point to affect as the source of modernism's psychic contamination. Departing from early psychoanalytic theories of suicide, and merging fin-de-siècle crowd theories, legal and clinical studies, and recent theories on the circulation of affect, this dissertation analyzes how physical crowding comes to precipitate a breakdown of psychic boundaries, threatening notions of identity and autonomy that the act of suicide sometimes paradoxically reaffirms. Moving from New York, to London, and finally to the culture capitals of continental Europe, an increasingly cosmopolitan engagement reveals affect's capacities to infect and overwhelm the individual, resulting in suicides that instigate progressively more collateral damage and that articulate the self as highly permeable, likely to be endangered by the contagious psychic and bodily states of others.