Before the emergence of the global COVID-19 pandemic, _immunity_ had become a relatively abstract concept in the industrialized world. For those who had easy access to health care and functioning immune systems, formerly life-threatening diseases like measles, polio, diphtheria, and influenza had been reduced to acronyms associated with vaccine schedules: MMR, IPV, DTaP, and Hib, among others. And yet even as the vaccine era dominated public health responses to infectious diseases, pockets of vaccine resistance persisted, and new diseases began to loom large in the cultural imagination: Zika, West Nile, SARS, Ebola, H1N1, and HIV/AIDS seemed to elude treatment and cure, raising the specter of global apocalypse. By mid-2020, COVID-19 embodied the worst fears and greatest promise of modern public health: a contagious disease that spread rapidly through casual social interactions but for which an effective vaccine could be developed, tested, and deployed in record time. By early 2021, the promise that vaccination would mean a quick return to prepandemic life was shattered: even as tens of millions of people were vaccinated in the United States alone, suspicion of governments and private corporations hardened into persistent anti-vaccine conspiracy theories. As vaccination rates plateaued, so did resistance to the SARS-CoV-2 virus....