Vaccines are one of the greatest inventions of the last 150 years. They've all but eradicated deadly diseases like smallpox, polio, and measles from most of the world. The same vaccines that allowed civilization to flourish in the twentieth century, however, have become a political hot button in the twenty-first. What changed? It's possible that a whole generation grew up without witnessing firsthand the horrors of deadly contagious disease on children, and so they never understood the value of vaccination.