The billionaire businessman fears bio-terrorism poses an even bigger threat than a nuclear war.
He believes terrorists could cook up and unleash catastrophic strains of super-infectious diseases such as small pox.
And once the deadly bugs start to spread it could trigger a global pandemic.
Mr Gates has urged world leaders to prepare in “the same way we prepare for war” – as he expressed his concern about a potential outbreak.
And he told how a “fast-moving airborne” disease could decimate 3 million in less than a year.
“All these advances in biology have made it far easier for a terrorist to recreate smallpox, which is a highly fatal pathogen, where there is essentially no immunity remaining at this point,” he told the Daily Telegraph ahead of a speech at the Royal United Services Institute in London last night.
“When you are thinking about things that could cause in excess of 10 million deaths, even something tragic like a nuclear weapons incident wouldn’t get to that level.
“So the greatest risk is from a natural epidemic or an intentionally caused infection bioterrorism events.”
His co-panellist, Norwegian Prime Minister Erna Solberg, backed Mr Gates: “Disease and violence are killing fewer people than ever before, but it’s spreading more quickly.
“We have forgotten how catastrophic those epidemics have been.”
And World Health Organization exec Peter Salama added: “It’s not if, but when.
“We need to ramp up our preparedness.”
Mr Gates added: “Natural epidemics can be extremely large.
“Intentionally caused epidemics, bio-terrorism, would be the largest of all.
“With nuclear weapons, you’d think you would probably stop after killing 100 million.
“Smallpox won’t stop. Because the population is naive, and there are no real preparations.
“That, if it got out and spread, would be a larger number.”
The philanthropist, 61, insisted fighting poverty is the best way to combat an outbreak.
“We ignore the link between health security and international security at our peril,” he said.
And Mr Gates, who is worth more than $85 billion, insisted: “The fact that a deadly global pandemic has not occurred in recent history shouldn’t be mistaken for evidence that a deadly pandemic will not occur in the future.”