This recent article in Nature blew me away. Its headline, “The Staggering Success of Vaccines,” says it all. Over the past 50 years, the measles vaccine alone has saved nearly 94 million lives. As one health expert says in the article, “Vaccines are one of humanity’s great achievements in terms of having furthered the lifespan and life quality for humanity in the past 50 years.”

Immunizing children and supporting other health-related efforts—like fighting malaria, HIV, and tuberculosis—is the biggest focus of my philanthropy. I’m moved to act by the fact that roughly 5 million children under the age of 5 die every year, the vast majority of them in poor countries. Meanwhile, lifesaving tools like vaccines and medicines get developed for diseases that affect those who can pay, and not for ones that mostly affect people in poorer countries. The inequity is horrible.

As a grandfather, I can’t imagine how awful it would be to lose a child, yet millions of families suffer that tragedy every year simply because of where they live. Fixing this disparity is the chief reason the Gates Foundation exists. I go to work each day thinking about how I can help close the gap.