Conspiracy theories have gained much academic and media attention recently, due to their large impact on public events. Crisis informatics researchers have examined conspiracy theories as a type of rumor. However, little is known about how conspiracy theories are produced and developed on social media. We present a qualitative study of conspiracy theorizing on Reddit during a public health crisis–the Zika virus outbreak. Using a mixed-methods approach including content analysis and discourse analysis, we identified types of conspiracy theories that appeared on Reddit in response to the Zika crisis, the conditions under which Zika conspiracy theories emerge, and the particular discursive strategies through which Zika conspiracy theories developed in online forums. Our analysis shows that conspiracy talk emerged as people attempted to make sense of a public health crisis, reflecting their emergent information needs and their pervasive distrust in formal sources of Zika information. Practical implications for social computing researchers, health practitioners, and policymakers are discussed.