More than a decade ago, the two of us read an article about the millions of children who were dying every year in poor countries from diseases that were long ago eliminated in this country. One disease we had never even heard of—rotavirus—was killing literally half a million kids each year. We thought: That''s got to be a typo. If a single disease were killing that many kids, we would have heard about it, because it would have been front-page news. But it wasn’t a typo.

We couldn''t escape the brutal conclusion that—in our world today—some lives are seen as worth saving and others are not. We said to ourselves: "This can’t be true. But if it is true, it deserves to be the priority of our giving."

We sent the article to Bill''s father, Bill Gates Sr., with a note attached that said, "Dad, maybe we can do something about this." And he helped us get started.

We created the Gates Foundation in 2000 because we believe in the principle that every human life has equal worth. The life of an impoverished child in a developing country is as precious as the life of a middle-class kid in a developed one. A family struggling to make ends meet in an American inner city matters as much as a family thriving in a safe, suburban neighborhood. Today, billions of people never even have the chance to live a healthy, productive life. We want to help all people get that opportunity.

We know it can be done because this is a unique moment in history: Scientific and technological advances are making it possible to solve big, complicated problems like never before. If these advances are focused on the problems of the people with the most urgent needs and the fewest champions, then within this century billions of people will grow up healthier, get a better education, and gain the power to lift themselves out of poverty. Warren Buffett shares our sense of optimism, and we are deeply humbled by his decision to give a significant portion of his resources to the foundation.