As I began to study vaccines, the single biggest surprise came in the form of a chart I saw in a book that had researched the history of vaccines. I’ll never forget the confusion this chart created in my head. All my life I’d heard vaccines saved humanity from horrible things like smallpox and polio. Before vaccines, there was widespread death and destruction from disease. After the vaccines, all of the childhood illnesses that had plagued humanity for centuries disappeared—basically overnight. This is the chart that changed everything for me. It showed how many people were dying from a couple of different diseases over the last 150 years. I couldn’t believe what it suggested: every disease was getting less deadly AT THE SAME TIME.