For the moment, Bill Gates is no longer the world's wealthiest man.

But he didn't lose the title to Mexico's telecom titan Carlos Slim; he gave it away. And as a result, the businessman-turned-philanthropist can point to a different kind of scoreboard.

"Well, it's easiest to measure in the health work," Gates told me, "where over 5 million lives have been saved."

In a wide-ranging interview with Yahoo! and ABC News, the former head of Microsoft talked about how Steve Jobs' death affected him, his fix for American schools and his annual letter, which sets the priorities for one of the most generous charitable efforts in history.

With a pledge to give away 95 percent of Gates' personal wealth, the Gates Foundation claims to have granted more than $26 billion since 1994. While some of that money is devoted to improving U.S. education, roughly 75 percent goes to the poorest countries in the world, and Gates scoffs at the idea that the money would be better spent at home.

"Well, the question is, are human lives of equal value?" Gates said. "For the mother whose child dies in Africa, is that somehow less important, less painful? If we can save that life -- for very little [money], is that appropriate to do? And, in fact, we know that if we do save those lives, it can reduce the population growth. It can let them be on a path to graduate from receiving aid."

After the Gates Foundation's vaccination efforts in India, that nation reported only one case of polio last year. And while the foundation promises to fight on against preventable diseases, the top focus of this year's letter is agriculture and Gates' belief that without technology, farmers could never feed the world's exploding population.

He calls for further research into the creation of flood-and-drought-resistant crops through genetic engineering.

"It is hard to overstate how valuable it is to have all the incredible tools that are used for human disease to study plants," he writes. But the idea of genetically modified organisms, or GMOs, is loaded with controversy. Environmental groups worry that the practice could upset the food chain, leading to the spread of disease, "superweeds" and mutant insects. Switzerland, Peru and Ecuador are among the handful of countries that have banned the cultivation of GMOs.

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To the dismay of GMO opponents, public records reveal that the Gates Foundation recently spent $27 million to buy 500,000 shares of Monsanto, the agribusiness giant with labs devoted to improving on nature to boost crop yields. And while Gates avoids the words "genetically modified" in his letter, he defends the idea when pressed.

"Over time, yes, countries will need to look at specific GMO products like they look at drugs today, where they don't approve them all. They look hard at the safety and the testing. And they make sure that the benefits far outweigh any of the downsides."

Aside from the environmental concerns, England's Prince Charles was among those who blamed a rash of farmer suicides in India on the higher cost of GMO seeds. But Gates insisted that his foundation's partners are not out to exploit developing nations.

"There's absolutely no payments, no royalties of any kind. It's just like in medicines. ... We go to the big companies who don't expect to make profits from the poorest billion and say: 'Will you help us?' And so they donate it."

Back in America, Gates is renewing his push toward "peer-reviewed" teaching as the key to reforming education. Since the best -- and worst -- teachers often operate in a bubble, he suggests training an elite group to roam from class to class to share what works and what doesn't.

"You take at least 2 percent of the teachers, train them very well and have them do structured visitations," he said. "And they tell the teacher, 'OK, you were good at this, but you didn't engage these kids very well. You didn't create discussion here. You didn't explain why a kid would wanna know this thing,' and help those teachers improve."

And Gates also reflected on the passing of Steve Jobs. Weeks before the Apple founder died, Gates paid an unannounced visit to the home of his sometime friend and longtime rival.

"He and I always enjoyed talking. He would throw some things out, you know, some stimulating things. We'd talk about the other companies that have come along. We talked about our families and how lucky we'd both been in terms of the women we married. It was great relaxed conversation.

How did Jobs' death affect him? "Well, it's very strange to have somebody who's so vibrant and made such a huge difference and been ... kind of a constant presence, to have him die," Gates said. "It makes you feel like, 'Wow, we're getting old.' I hope I still have quite a bit of time for the focus I have now, which is the philanthropic work. And there's drugs we're investing in now that won't be out for 15 years -- malaria eradication, I need a couple of decades here to fulfill that opportunity. But, you know, it reminds you that you gotta pick important stuff, because you only have a limited time."

For more from Bill Weir's exclusive interview with Bill Gates watch "Nightline " Tuesday night at 11:35 p.m. ET on ABC.

The Bill and Melinda Gates Foundation supported, in part, last year's ABC NEWS initiative BE THE CHANGE: SAVE A LIFE , which focused on health care in some of the poorest areas of the world.