Ages: 54 and 45
Previous TIME 100 Appearances: 3 and 1, respectively
For years, Microsoft co-founder Bill Gates, along with his wife Melinda, has been giving away his fortune. In the past, the pair focused on issues like education and third-world poverty. Now they're accelerating work in the administration of vaccines, angling to spend $10 billion over the next decade to prevent diseases such as pneumonia and diarrhea.