As a species we have doubled our life expectancy in just one hundred years. All the advances of modern life - the medical breakthroughs, the public health institutions, the rising standards of living - have given us each about twenty thousand extra days on average. There are few measures of human progress more astonishing than our increased longevity. This book is Steven Johnson's attempt to understand where that progress came from. How many of those extra days came from vaccines? What are the forces that now keep us alive longer? Behind each breakthrough lies an inspiring story of innovation.