Collecting blood samples from individuals recruited into clinical research projects in sub-Saharan Africa can be challenging. Strikingly, one of the reasons for participant reticence is the occurrence of local rumors surrounding "blood stealing" or "blood selling." Such fears can potentially have dire effects on the success of research projects--for example, high dropout rates that would invalidate the trial's results--and have ethical implications related to cultural sensitivity and informed consent. Though commonly considered as a manifestation of the local population's ignorance, these rumors represent a social diagnosis and a logical attempt to make sense of sickness and health. Born from historical antecedents, they reflect implicit contemporary structural inequalities and the social distance between communities and public health institutions. We aim at illustrating the underlying logic governing patients' fear and argue that the management of these beliefs should become an intrinsic component of clinical research.