The biggest global risk Melinda Gates can imagine is "most definitely" a bioterrorism attack, she said in an interview at South by Southwest.

Melinda and Bill Gates have both repeatedly warned that the world is unprepared for an infectious disease outbreak, and experts agree that the risk is high.

If you look at recent disease outbreaks around the globe, you can see how far we have to go in terms of preparation.

The biggest global risk that Melinda Gates can imagine within the next 10 years is "most definitely" a bioterrorism attack.

"A bioterrorist event could spread so quickly, and we are so unprepared for it," she told Vox founder Ezra Klein in an interview at South by Southwest over the weekend.

"Think of the number of people who leave New York City every day and go all over the world — we're an interconnected world."

It's scary enough, she said, that she doesn't like to talk about it.

But she and Bill have been warning people that one of the biggest threats out there is one of the oldest: infectious disease, which can emerge naturally or be human-made (as in the case of bioterrorism).

As the two wrote in their recently released "Goalkeepers" report, disease — both infectious and chronic — is the biggest public health threat the world faces in the next decade. And although Bill Gates said on a press call at the time that "you can be pretty hopeful there'll be big progress" on chronic disease, we are still unprepared to deal with the infectious variety.

Bill Gates has repeatedly stated that he sees a pandemic as the greatest immediate threat to humanity on the planet.

"Whether it occurs by a quirk of nature or at the hand of a terrorist, epidemiologists say a fast-moving airborne pathogen could kill more than 30 million people in less than a year," Gates wrote in an op-ed for Business Insider last year. "And they say there is a reasonable probability the world will experience such an outbreak in the next 10-15 years."

Plague bacteria is one of the oldest bioweapons in the world. Infected corpses used to be catapulted over city walls. National Institute of Allergy and Infectious Diseases (NIAID)

A very real risk

Gates is right about the gravity of that threat, according to experts in the field.

George Poste is an ex officio member of the Blue Ribbon Study Panel on Biodefense, a group created to assess the state of biodefense in the US.

"We are coming up on the centenary of the 1918 influenza pandemic," he told Business Insider. "We've been fortunately spared anything on that scale for the past 100 years, but it is inevitable that a pandemic strain of equal virulence will emerge."

The 1918 pandemic killed approximately 50 million people around the globe, making it one of the deadliest events in human history.

David Rakestraw, a program manager overseeing chemical, biological and explosives security at Lawrence Livermore National Laboratory, and Tom Slezak, the laboratory's associate program leader for bioinformatics, also agree with Gates.

"Both natural and intentional biological threats pose significant threats and merit our nation’s attention to mitigate their impact," they told Business Insider in an email.

It's possible that a major outbreak could be intentionally created as the result of a biological weapon, but Poste thinks a serious bioterrorism attack is less likely due to the complexity of pulling something like that off.

It's very likely, however, that a highly dangerous disease would naturally emerge — and the consequences of that pandemic would be just as severe.

In New York City's hazard mitigation plan, the city indicates that a bioterror attack could have an impact on a similar scale as that of a nuclear weapon. And they say that the likelihood of bioterror attack is far greater.

Regardless of how a disease starts to spread, preparedness efforts for pandemics are the same, according to Poste. And the recent outbreaks of Zika and Ebola have highlighted the need for more heightened disease surveillance capabilities. We're still getting a handle on the health effects of Zika — and it seems like the mosquito-borne disease may be even more severe than we thought.

Experts have long advocated for better ways to recognize emerging threats before they become epidemics or pandemics. Poste also said we need to improve rapid diagnostic tests and get better at developing new therapeutics and vaccines — something highlighted as a weakness in the Gates' "Goalkeepers" report as well.

To prepare fpr a bioterrorism event, Melinda Gates said we should have an organization like the CDC but with an exclusive focus on bioterror, creating safety standards and monitoring the globe.

Until that happens, that threat remains far more real than many of us realize.