In controlled human infection (CHI) research, researchers intentionally expose people to pathogens to gain scientific insights. During the COVID-19 pandemic, proposals to use CHI research to speed up vaccine development captured public imagination and bioethicists’ attention, with more papers written on the ethics of CHI research than ever before.Intentionally Infecting Humans: Is it Ethical? argues that ongoing ethical controversy about CHI research is fuelled by fundamental confusion. This book draws on past uses and abuses of CHI research, the experiences of volunteers, and social psychology research to help explain the status of these studies in the public imagination. It then provides an ethical framework for these studies, demonstrating that the biggest ethical challenge with this research is when scientists determine how to infect humans for the first time in a novel CHI model. Once a model has been shown to be safe and reliable, it is much less controversial to use CHI models in studies that can, for example, test new vaccines and treatments. Furthermore, given the time it takes to create a safe and reliable model, CHI research is not the "silver bullet" for vaccine development some promised during the COVID-19 pandemic but still may be a smart investment in the longer term. Distinguishing types of CHI research that raise more and less ethical concern also helps clarify when CHI research can be used by low- and middle-income countries embarking on new research programs, and for populations traditionally considered "vulnerable." The book leaves the reader with a richer understanding of the history that feeds into ongoing controversies over CHI research, ways that CHI research could be deployed ethically to enhance preparedness for future pandemics and address neglected diseases, and ideas to help resolve longstanding debates in research ethics more generally.