Ezra Klein sat down with billionaire Bill Gates for Vox and asked him what he is most afraid of. Initially, the tech giant listed a few things that cross his mind but have a "low probability" of occurrence during his lifetime -- a big volcanic explosion, gigantic earthquake, asteroid impact, or nuclear war. But it is the possibility of a widespread epidemic that has Gates truly worried:

I rate the chance of a widespread epidemic, far worse than Ebola, in my lifetime as well over 50 percent.

Pointing to the spike in the number of deaths in the 20th Century, particularly World War I and World War II with deaths estimated at 25 million and 65 million respectively, Gates points out a spike often ignored between the two wars that is estimated to have claimed 50 million lives. This was the outbreak of the Spanish Flu in 1918.

For purposes of research, Gates funded the Institute for Disease Modeling which utilizes computer simulation to pinpoint crucial areas where a possible outbreak epicenter could be to find out how resources could be allocated to those areas to help control the spread of disease. Modeling a similar outbreak to the 1918 flu epidemic and taking into account the mass international transit that is available today, the institute estimated worldwide infection within days of an outbreak and over 33,000,000 deaths in under a year.

"That is very eye-opening," Gates said.

After witnessing the most recent Ebola outbreak, Gates declares that neither the world, nor the United States, is ready to handle a more severe and infectious epidemic, especially one that would spread faster than Ebola. And so he suggests that the federal government invests more wisely in this likely scenario an amount far less than what the U.S. currently spends on national security:

This is the greatest risk of a huge tragedy. This is the most likely thing, by far, to kill over 10 million excess people in a year. We don't need to invest nearly what we do in military preparedness. This is something where less than a billion a year on R&D, medical surveillance, standby personnel, cost training the military so they can play a role in terms of all the logistics here. This can be done and we may not get many more warnings like this one to say, 'Okay, it's a pretty modest investment to avoid something that really, in terms of the human condition, would be a gigantic setback.