Philanthropists Bill and Melinda Gates made headlines last month as they both penned op-eds about their investments in global health — which total a cool $10 billion in the last two decades — and the need for continued global support in achieving Global Goal 3 on good health for all.

Now they’re addressing the ups and downs of global health in their annual letter. Last year’s letter highlighted the toughest questions they get asked, but this year’s letter outlines the top “things they didn’t see coming” when it came to development and global health initiatives.

1. “Africa is the youngest continent.”

In most parts of the world, people are living longer, and the global median age is increasing. But in Africa, the median age is only 18 — and it is estimated that the number of young people there will continue to increase, the Gateses noted.

One of the reasons, Bill explained, is that annual birth rates continue to rise in poor regions of sub-Saharan Africa.

“This can be either an asset or a source of instability,” he wrote. “Melinda and I believe that the right investments will unlock the continent’s enormous potential. Young Africans will shape the future of not only their own communities but the entire world.”

Melinda pointed out that if sub-Saharan Africa committed to investing in young people, it could double its involvement in the global workforce by 2050. She pointed to investment in girls’ education as the key to achieving this, as educated girls are healthier, wealthier, and better equipped to raise healthy children.

Take Action: Stand With Every Woman, Every Child: Ask World Leaders to End Preventable Deaths

2. “At-home DNA tests can find serial killers — and could also help prevent premature birth.”

Last year, the Golden State Killer was caught thanks to genetic test results, but these DNA tests also led scientists to discover a potential link between preterm labor and a mineral called selenium, Bill noted.

“Some people have a gene that prevents them from processing selenium properly. The 23andMe study (which our foundation helped fund) found that expectant mothers who carry that gene were more likely to give birth early,” he explained.

Further research is still needed to see how this is truly connected to premature births, but it could be significant in determining a solution to premature births.

3. “The world will build an entire New York City every month for 40 years.”

By 2060, the world’s building stock is expected to double, but Bill also points out that agriculture accounts for a significant portion of greenhouse gases (24%).

His point?

“If we’re going to solve climate change, we need to get to near-zero emissions on all the things that drive it — agriculture, electricity, manufacturing, transportation, and buildings,” he wrote.

4. “Data can be sexist.”

The Gateses explain that there is little data available on girls and women in developing countries, but also that the data that is available is also often sexist.

"We like to think of data as being objective, but the answers we get are often shaped by the questions we ask. When those questions are biased, the data is too," Melinda wrote.

She pointed to the example of data on reproductive health of women in developing countries. Because these women are often wives and moms, that’s what the little data available is all about — there’s no information on things like how much they earn or own.

“When such flawed data is all you have to go on, it’s easy to undervalue women’s economic activity — and difficult to measure whether women’s economic condition is improving,” she wrote.

5. “You can learn a lot about processing your anger from teenage boys.”

The Gateses touched on the link between poverty and mass incarceration and their visit to a Georgia state prison, but Bill wrote most about his time spent at a Becoming a Man (BAM) meeting and the boys he met there.

BAM is a program that helps young men in areas vulnerable to crime and gangs discuss their emotions and refine their decision-making skills. He said he learned a lot about coping with anger from participating in their meeting.

“This particular BAM group had been together for a year, and it showed. I was touched by the respect they had for each other and the intimacy they allowed themselves. I left thinking: This is how every classroom in the world should feel,” he wrote.

6. “There’s a nationalist case for globalism.”

This year’s letter noted that there’s a case to be made for nationalism in its belief relating to one’s love of their country. Melinda points out that an investment in foreign aid can mean an investment at home too — increased stability abroad means better security at home, and she cited responses to Ebola as a good example in supporting global health security.

"There is nothing about putting your country first that requires turning your back on the rest of the world. If anything, the opposite is true," Melinda wrote.

7. “Toilets haven’t changed in a century.”

Bill and Melinda organized a toilet fair in Beijing last year, where they saw firsthand the next generation of toilets. These innovations are outwardly similar to today’s toilets, but with some significant differences under the hood.

“They’re essentially tiny treatment plants capable of killing pathogens and rendering waste safe on their own. Many of them even turn human feces and urine into useful byproducts, like fertilizer for crops and water for handwashing,” Bill wrote.

These toilets could save millions of lives, which is why the Gates Foundation is investing in research and development of toilets to make them more affordable.

8. “Textbooks are becoming obsolete.”

The Gateses point out that educational software is changing the way people learn around the world — not eliminating teachers, but rather working to supplement them.

Software also adapts to who and where students are, which is especially beneficial for non-traditional students like older students with jobs or people with children.

Digital learning tools can make college more affordable, convenient and effective, Melinda explained.

9. “Mobile phones are most powerful in the hands of the world’s poorest women.”

"For the world's most marginalized women, a mobile phone doesn't just make their old life more convenient; it can help them build an entirely new life," Melinda wrote. "That’s because connectivity is a solution to marginalization."

She wrote about her time spent with Nikmah, a woman from Indonesia who now makes money through Go-Jek, an app that connects people to rides, food deliveries, and more. It helps Nikmah make and control her own money.

But there is a gender gap in internet and phone ownership that needs to be addressed for this potential to be truly unlocked.

In fact, Melinda noted women are almost 40% less likely than men to have used the internet, according to a study of 10 countries across Africa, Asia, and South America.

Every year comes with ups and downs, and 2018 was no different, but the Gateses ended their note by proclaiming their optimism in the future.

"We’ve found that optimism can be a powerful call to action," the letter concluded. "And it has a multiplier effect: The more optimists there are working for a better future, the more reasons there are to be optimistic."

Disclosure: The Bill and Melinda Gates Foundation is a funding partner of Global Citizen.

Editor's note: This piece has been updated to include a disclosure that the Bill and Melinda Gates Foundation is a funding partner of Global Citizen. We regret the oversight.