Vaccination is one of humanity's most effective and greatest discoveries. Infections like the plague, smallpox and other deadly diseases have affected and killed people for thousands of years, but the invention of vaccines forever changed our relationship with these diseases. More recently the urgency of developing an effective vaccine during the COVID-19 pandemic brought vaccination to the public's attention. Simmering tensions around vaccine hesitancy, misinformation and mistrust of science came to the forefront. Although an earlier form of protection against infectious diseases has been practiced for a long time, vaccines have only been around for 200 years. Why We Need Vaccines explores the history of vaccine discovery, the science of how vaccines work and the public-health achievements that vaccines have made possible. It also discusses vaccine mandates and inequality in access to vaccines on local and global scales. It challenges young readers to take responsibility for themselves, their families and their communities so we can all be part of the solution to take down infectious diseases.