Mobilization researchers have recently sought mechanisms that produce movement outcomes by reshaping relations among groups involved in contentious politics. Environmental mechanisms, which externally influence conditions of contention, are particularly salient in health-related movements where biological and ecological factors are implicated. In this article I conceptualize an environmental mechanism associated with collective responses to epidemic and eruptive social problems: the mechanism of crisis—an unforeseen and rapidly spreading threat that (1) motivates transgressive contention by those threatened, (2) elevates challengers through alliances with powerful groups that share their perspective, and (3) facilitates policy change by temporarily undermining officials' credibility. I illustrate this mechanism in the case of syringe services programs for HIV prevention in California. Using archives, newspapers, and indepth interviews, I show that the inconsistent spread of syringe services followed the development and deterioration of an HIV/AIDS-related crisis. This mechanism also explains resurgent attention to syringe services amid the ongoing overdose crisis.

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