How viruses emerge to cause pandemics, how our immune system combats them, and how diagnostic tests, vaccines, and antiviral therapies work. Throughout history, humans have contended with pandemics. History is replete with references to plagues, pestilence, and contagion, but the devastation wrought by pandemics had been largely forgotten by the twenty-first century. Now, the enormous human and economic toll of the rapidly spreading COVID-19 disease offers a vivid reminder that infectious disease pandemics are one of the greatest existential threats to humanity. This book provides an accessible explanation of how viruses emerge to cause pandemics, how our immune system combats them, and how diagnostic tests, vaccines, and antiviral therapies work—concepts that provide the foundation for our public health policies. The authors, both experts in immunology, interweave explanations of scientific principles and ongoing efforts to combat COVID-19 with stories of the people behind the science. They recount the eradication of smallpox—the greatest accomplishment of vaccines and public health; discuss microbial pathogens; and describe what we know about modern pandemics, including how they spread, how they can be contained, and how they can be treated. They do not offer prescriptions for what to do, but equip readers to be informed participants in debates about how to create a more pandemic-resilient world. We do not have to repeat the mistakes made with COVID-19.