Beginning with smallpox and concluding with an overview of the COVID-19 pandemic, Brown traces the evolution of vaccines and examines deadly diseases such as measles, polio, anthrax, rabies, cholera, and influenza. The book is narrated by Lady Mary Wortley Montagu, who historically popularized inoculation in England in the early 18th century. Brown covers the science behind how our immune systems work, the discovery of bacteria, and major achievements from scientists like Louis Pasteur, Antonie van Leeuwenhoek, and Edward Jenner, the "father of immunology." Brown also tackles the public and political response to vaccination throughout history, addressing head-on the anti-vaccination movement and debunking false claims that vaccines cause autism.