It's the death of children.
Gates is in New York talking to UN about the Millennium Development Goals (MDGs), which is a set of priorities for improving life in poorer nations.
Ahead of his talk at the UN, he spoke with a few reporters, including ourselves.
At the end of his talk he was asked about the progress made on the MDGs. He says he's pretty happy overall with how life has improved, but the top achievement is the reduction in child deaths.
Here's Gates:
"Child mortality, we cut it half. We're talking about 12 million a year. Remember there's always mis-perspective that the latest natural disaster or unrest is the primary source of misery. The primary source of misery in the world is the ongoing mothers whose children die. That was 12 million back in 1990. Now, we'll be around 6 million by the end of this time period. That's just such a phenomenal thing. Ask people on the street, "Hey, what bad things are going on?" They'll say, "Well maybe Syrian refugees..." It's just not well known and that to me is the top achievement of this entire period."
Unrelated, but, still interesting, he said that while a lot of people think having a good, democratic country results in better healthcare for the poor, that's not always the case. He was asked about why Ethiopia succeeded despite a political system that isn't great:
In poor countries there's a negative correlation between democracy ratings and saving children's lives. Negative correlation. That's been true since 1950s, 1960s, 1970s, 1980s. The way these factors interplay to each other is fairly complex. The Ethiopia story is a primary health care story. Meles supported Tedros. The donors came in, they hired these, put out 15,000 health posts, 30,000 trained community health workers - it's a work in progress, it's not Vietnam or Rwanda. Rwanda, now there's a great health system. Ethiopia benefited from having such a bad baseline. You want to get to a certain level, you better be doing a really bad job in 1990s, it is very helpful. Ethiopia is a very poor country, so this primary healthcare they're in the process of building ... it's quite impressive. They've built a system, people show up, the supply chain largely works.