Here we identify approximately 40,000 healthy human volunteers who were intentionally exposed to infectious pathogens in clinical research studies dating from late World War II to the early 2000s. Microbial challenge experiments continue today under contemporary human subject research requirements. In fact, we estimated 4,000 additional volunteers who were experimentally infected between 2010 and the present day. We examine the risks and benefits of these experiments and present areas for improvement in protections of participants with respect to safety. These are (...) the absence of maximum limits to risk and the potential for institutional review boards to include questionable benefits to subjects and society when weighing the risks and benefits of research protocols. The lack of a duty of medical care by physician–investigators to research subjects is likewise of concern. The transparency of microbial challenge experiments and the safety concerns raised in this work may stimulate further dialogue on the risks to participants of human experimentation. (shrink)