The term ''blood activism'' describes the range of collective challenges that arose among victims of Canada's tainted-blood scandal in the 1990s. This article examines the emergence of blood activism in Canada from the perspective of social movement theory, paying particular attention to the tensions between victims who contracted HIV through tainted blood and those who contracted Hepatitis C, the so-called ''forgotten victims'' of the tragedy. This study discusses how changes in the ''political opportunity structure''—loosely defined in the literature as aspects of the movement's external environment—influenced the nature of political action pursued by victims of tainted blood, the negotiation of the movement's collective identity and policy outcomes.