Bill Gates, who has used part of his fortune to tackle some of the world's major problems, thinks big. He asks the question: What would be the most likely cause of the deaths of 10 million people in the next twenty years? He has considered many possibilities — "big volcanic explosion, gigantic earthquake, asteroid"—but tells Vox.com that the probability is "very low."

Even war doesn't cause Gates to lose too much sleep, and the possibility of a devastating nuclear war isn't his top concern.

After considering all of the possibilities, Gates' top worry is a pandemic disease, which he says is the most predictable threat to the human race, and we're not ready.

"The world is simply not prepared to deal with a disease — an especially virulent flu, for example — that infects large numbers of people very quickly," Gates wrote on his blog. "Of all the things that could kill 10 million people or more, by far the most likely is an epidemic.

"Look at the death chart of the 20th century," he tells Vox.com, "I think everybody would say there must be a spike for World War I. Sure enough, there it is, like 25 million. And there must be a big spike for World War II, and there it is, it's like 65 million. But then you'll see this other spike that is as large as World War II right after World War I, and most people, would say, 'What was that?'

"Well, that was the Spanish flu."

The recent Ebola epidemic was a wakeup call. "As awful as it is, Ebola spreads only through physical contact, and by the time patients can infect other people, they are already showing symptoms of the disease, which makes them relatively easy to identify," Gates wrote in the New York Times.

"Other diseases — flu, for example — spread through the air, and people can be infectious before they feel sick, which means that one person can infect many strangers just by going to a public place," he continued. "We’ve seen it happen before, with horrific results: In 1918, the Spanish flu killed more than 30 million people. Imagine what it could do in today’s highly mobile world."

A natural epidemic is most likely to start in a poor country, where there are no early-warning systems in place, Gates said. "Even once the Ebola crisis was recognized last year, there were no resources to effectively map where cases occurred, or to use people’s travel patterns to predict where the disease might go next."

Gates said that doctors and other personnel should have been galvanized within days instead of weeks, and future epidemics should have more accurate methods for tracking cases. "We need a global warning and response system for outbreaks," he wrote. He also considers it vital to invest in tests that will diagnose disease rapidly and accurately, and to have an expedited process for approving new drugs during a crisis.

Gates and his wife head the Bill & Melinda Gates Foundation, which has tackled child mortality, malaria, and polio, and the results are measurable and impressive. According to Vox.com, since 1990, child mortality has been cut in half.

"Melinda and I remain committed to improving the health of the poorest 2 billion," Gates writes on his blog. "The good news is, many of the steps required to save lives in poor countries — such as strengthening health systems — also improve the world’s ability to deal with epidemics.

"So I’m optimistic that we can solve this problem. Making the right investments now could save millions of lives."