TIME: How did you decide to give most of your fortune back to the world?
MELINDA: It was clear to us that we didn't want to leave it to our kids. When we started to look at where the largest inequities [are], global health really stood out, because by every measure, if you can improve people's lives through health, you improve all measures of society.
TIME: There are so many inequities. How do you go about deciding what to fund?
BILL: It's the diseases that get no attention. We look at the top 20 and make sure that there's money for...