Vaccines work wonders. They prevent disease from striking, which is better than treating it after the fact. They are also relatively cheap and easy to deliver. Yet millions of children do not get them. This has always been stunning to me. When we started the Gates Foundation 15 years ago, we assumed that all of the obvious steps were already being taken, and that we would have to go after the expensive or unproven solutions. In fact, delivering basic vaccines is still one of our top priorities.