Public-health phenomena aren't typically classified as "beautiful," but Bill Gates knows when he sees a good thing.

On February 16, the billionaire philanthropist tweeted a chart describing the decline of child-mortality figures between 1990 and 2015. Among children 5 years and younger, advances and investments in medical science have led to 6 million fewer deaths around the world.

Gates captioned the tweet simply, "This is the most beautiful chart in the world."

The tweet coincided with the February 14 release of the Bill and Melinda Gates Foundation's annual Gates Letter, a forward-looking post on the Gates Notes blog that highlights how far the world has come in eradicating disease and where the greatest challenges still lie.

This year's letter was a tribute to fellow entrepreneur and philanthropist Warren Buffett, a longtime friend and mentor to the Gateses. The letter, entitled "Dear Warren," celebrated Buffett's enormous contributions to the Gates Foundation over the years.

In 2006, the Berkshire Hathaway founder gave away the bulk of his multi-billion-dollar fortune in the pursuit of curing deadly diseases and eliminating chronic poverty.

Part of that mission — and its resultant success — was highlighted in Gates' recent tweet. With more than 50% declines in child mortality over the past 25 years, Gates estimates that some 122 million kids' lives were saved during that time.

Many of the greatest threats to children are preventable with access to simple, affordable interventions, according to the World Health Organization. Kids in developing countries die all too frequently from preterm-birth complications, pneumonia, chronic diarrhea, and malaria. The WHO also estimates that 45% of early deaths are due to malnutrition.

"Kids in rich countries don't die from these things," Gates wrote in the letter. He said he saw the poverty firsthand when he and Melinda visited Africa in the 1990s. "The children in Africa were dying because they were poor. To us, it was the most unjust thing in the world." (emphasis his)

"Saving children's lives is the goal that launched our global work," Melinda added. "It's an end in itself."