Co-founder of Microsoft and co-chair of the Bill and Melinda Gates Foundation Bill Gates believes that widespread infectious disease is the greatest threat to humankind, which he thinks is not prepared, the Time reported.

In the next 20 years, is it likely that nuclear war, gigantic earthquakes or asteroids could kill 20 million people? Bill Gates doesn't think so.

But he did tell Vox that such numbers could be felled by a major outbreak of disease — something the 59-year-old billionaire believes has a "well over 50%" chance of happening in his lifetime.

"The Ebola epidemic showed me that we're not ready for a serious epidemic, an epidemic that would be more infectious and would spread faster than Ebola did. This is the greatest risk of a huge tragedy," Gates said, claiming that a serious epidemic could kill more than 10 million people a year.