The Microsoft Founder, who was speaking ahead of speech at the Royal United Services Institute in London (RUSI), warned that an outbreak of a lethal respiratory virus like smallpox would be more dangerous than even a nuclear attack.
Mr Gates, whose charitable foundation funds research into quickly spotting outbreaks, said it was more important than ever to help foreign countries monitor diseases to prevent a global tragedy.
“Bioterrorism is a much larger risk than a pandemic,” he said.
“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.
“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.
“Whether the next epidemic is unleashed by a quirk of nature or the hand of terrorist, scientists say a fast-moving airborne pathogen could kill more than 30 million people in less than a year. So the world does need to think about this.”
Last year the Nuffield Council of Bioethics warned that ‘garage scientists’ could unleash dangerous genetically modified organisms into the environment using unregulated technology which is already available online.
Chemistry kits which allow genetic editing can already be bought online for under £100.
Scientists are concerned that a new technique, called Crispr, is now so cheap and widely available that amateurs will start experimenting at home, or in school labs.
The technique works like genetic scissors to cut away DNA code and replace it with new genes. It has been hailed as one the most significant scientific breakthroughs in recent years, but there are fears that in the wrong hands, the procedure could unleash dangerous strains of bacteria or other organisms. Kits to make E.coli resistant to antibiotics are already for sale on the internet.
Mr Gates said that today’s widespread global travel mean that a future pandemic could be even more deadly than the Spanish Flu outbreak in 1919 which killed up to 100 million people.
“We will have epidemics in the next 20 years far worse than the ebola epidemic, or the Zika epidemic and there is some chance it would be a form of flu,” he said.
“Something that is human-to-human respiratory that is like a measles or a flu or smallpox, that you need just one person on the bus or plane or the airport and you get huge things. A health crisis somewhere is a health crisis everywhere.
“So the scariest thing is something like the 1919 flu which really spreads everywhere and because people are moving around more it’s easier for it to spread than back in 1919. If 1919 came back we have no immunity to that strain.”
Mr Gates urged Theresa May to maintain the 0.7 per cent GDP foreign aid pledge.
“The UK's foreign aid investments are, in fact, long term investments in the health and security of British citizens here at home," he added.
“A disease is more likely to become an epidemic when countries are unstable and have no functioning health system. We can’t build a wall to hold back the next global epidemic.”