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funder of vaccines, I was especially interested in what it took to deliver a vaccine over what people in the field call "the last mile"--the journey from a storage facility to a remote clinic and finally to a patient. The WHO had recently approved a new vaccine funded by the Gates Foundation to protect children from rotavirus. At the time, more than 400,000 children were dying of rotavirus every year. This clinic had received a shipment of the new rota vaccine, but there was a problem. To make a point, one of the health workers took a few of the vials and tried to put them a portable cooler. (These coolers are what vaccinators carry vaccines in when they go into the field.) The point of the demonstration was to show me a problem: The new vials didn't fit in the cooler. This may seem like a minor detail, but it's a big problem. Vaccines will spoil if they aren't kept cold--typically between 2 and 8 degrees C, or roughly 35 to 46 degrees F--during the trip from the factory to their final destination. If a health clinic can't keep the vials cold, the doses are spoiled. (Keeping them at the right temperature throughout the process is known as maintaining the cold chain.) The manufacturer of the rotavirus vaccine soon fixed the problem by changing the size of the vials, but it was a vivid illustration of a fundamentally important point about vaccines: Delivering them to every part of the world where they're needed is an enormous logistical challenge, and seemingly small decisions like the size of a container can throw everything off. The good news is that the cold chain and other hurdles in delivering vaccines have been solved in most parts of the world. Today, 85 percent of children get at least three doses of the