Bill and Melinda Gates are optimistic about the future, and they think you should be, too.

As the co-chairs of the Gates Foundation explain in their recent Gates Letter, an annual review of global progress, the world is doing a lot better than the public gives it credit for.

People aren't as poor, as sick, or as unequal as they used to be.

The letter includes a dizzying number of charts, graphs, and stats illustrating that point, but here are four that highlight some of the Gates' favorite leaps.

1. The polio vaccine has been incredibly effective in eradicating disease.

Image: Gates Letter

New polio cases haven't gone down much in the last 17 years — mostly because they're already so close to zero.

In the 64 years since Jonas Salk developed the first vaccine, countries have taken concerted efforts to get people inoculated. China saw its last cases in the mid-1990s and India about 20 years later.

In 2016, there were fewer than 40 new cases logged worldwide. Currently, just four countrieshave yet to eradicate it.

2. Contraceptive use is higher than it's ever been.

Image: Gates Letter

One of the most effective methods for ensuring economic and social success is female empowerment. In its 2015 Gates Letter, the Bill and Melinda Gates Foundation noted that investments in women's health result in stronger workforces and economies in general.

In many developing countries, fertility rates are extremely high. This is due in part to women bearing more children, because they know some will not survive infancy. But that keeps them in a cycle of poverty, as resources spread thin.

Contraceptives help break that cycle, letting women focus more on themselves and their families. Roughly 300 million women in the 69 poorest countries used some form of birth control in 2016 — a jump of 30 million from 2012.

3. Some 122 million kids' lives have been saved over the last 20 years.

Image: Gates Letter

Gates has called this the most beautiful chart in the world.

Since 1990, the number of kids dying before their 5th birthday has been cut in half, from 12.1 million down to 5.8 million, in 2015. This is largely due to advances and investments in simple, inexpensive measures in developing countries, including bed nets to prevent malaria and drilling wells for access to clean water.

The World Health Organization finds preterm birth complications and malnutrition play deciding roles in how children fare in their first years of life.

Gates has invested heavily in addressing the needs of kids ever since realizing the extent of the problem during a trip to Africa in the 1990s, the letter explains.

4. Extreme poverty is slowly going extinct.

Image: Gates Letter

Extreme poverty is defined as living on less than $1.90 a day, and for about 150 years (between 1820 and 1970) the number of people dealing with it continued to rise. By the end of the 1970s, more than 2 billion people could be classified as extremely poor.

People still seem to think the world is getting poorer, but the data show the number of people in extreme poverty has shrunk to 705 million from that 2-million high, Melinda Gates explains in the letter.

"The larger historical trends are toward greater inclusion and caring," she said. "We definitely see it in global health. Governments are prioritizing it. Citizens are supporting it. And scientists are migrating to it."