I. Bill Gates is an optimist.

Ask him, and he'll tell you himself. "I'm very optimistic," he says. See?

And why shouldn't Bill Gates be an optimist? He's one of the richest men in the world. He basically invented the form of personal computing that dominated for decades. He runs a foundation immersed in the world's worst problems — child mortality, malaria, polio — but he can see them getting better. Hell, he can measure them getting better. Child mortality has fallen by half since 1990. To him, optimism is simply realism.

But lately, Gates has been obsessing over a dark question: what's likeliest to kill more than 10 million human beings in the next 20 years? He ticks off the disaster movie stuff — "big volcanic explosion, gigantic earthquake, asteroid" — but says the more he learns about them, the more he realizes the probability is "very low."

Then there's war, of course. But Gates isn't that worried about war because the entire human race worries about war pretty much all the time, and the most dangerous kind of war, nuclear war, seems pretty contained, at least for now.

But there's something out there that's as bad as war, something that kills as many people as war, and Gates doesn't think we're ready for it.

"Look at the death chart of the 20th century," he says, because he's the kind of guy that looks at death charts. "I think everybody would say there must be a spike for World War I. Sure enough, there it is, like 25 million. And there must be a big spike for World War II, and there it is, it's like 65 million. But then you'll see this other spike that is as large as World War II right after World War I, and most people, would say, 'What was that?'"

"Well, that was the Spanish flu."

II. The most predictable threat in the history of the human race

No one can say we weren't warned. And warned. And warned. A pandemic disease is the most predictable catastrophe in the history of the human race, if only because it has happened to the human race so many, many times before.

In a 1990 paper on "The Anthropology of Infectious Disease," Marcia Inhorn and Peter Brown estimated that infectious diseases "have likely claimed more lives than all wars, noninfectious diseases, and natural disasters put together." Infectious diseases are our oldest, deadliest foe.

And they remain so today. "In a good year, flu kills over 10,000 Americans," says Dr. Thomas Frieden, director of the Centers for Disease Control and Prevention. "In a bad year, it kills over five times that. If we have a pandemic, it will be much worse. People think the H1N1 flu wasn’t so bad. But more than 1,000 American kids died from H1N1!"

Each new year seems to bring its own sensational candidate for the next pandemic. In 2014, of course, it was the Ebola outbreak — which killed more than 10,000 people, and sent much of America into hysterics. This year, a particularly infectious form of bird flu has ripped through 14 states, killing or forcing the slaughter of 39 million birds. Public health authorities are forcing the grisly massacre because the more birds around for the flu to infect, the more chances the flu has to mutate and reassemble itself into a form that can infect humans.

It isn't just the news that carries warnings. The culture is thick with our fear of infectious disease. Zombies, for instance, are everywhere — World War Z was a best-selling book and a blockbuster movie; The Walking Dead has become one of television's most popular shows. And zombies are a metaphor for infectious disease.

"When I was a kid, I watched AIDS go from an obscure, arcane curiosity to a global pandemic," Max Brooks, author of World War Z, told the CDC. "What drove me crazy was that unlike the Black Death or the Spanish Influenza, AIDS could have simply been stopped by a pamphlet: A couple dos and don’ts, a little education and clear-headed leadership and it might have ended up as a footnote in a virologists’ medical text. If that’s not zombies, I don’t know what is."

The CDC has even released a document titled "Preparedness 101: Zombie Apocalypse." The point, obviously, isn't that the CDC expects a zombie apocalypse around the corner; it's that since a zombie apocalypse is simply an infectious disease apocalypse, talking about how to avoid becoming a zombie is a safe way for people to talk about how to protect themselves from pandemic disease.

"When confronted with real anxiety, a lot of people shut down," Brooks said. "For them, planning for an actual crisis is just too scary, too paralyzing to think about. Make it a zombie attack, though, then there’s some psychological padding."

Pandemic disease is something our culture thinks about, knows about, fears. It's so topmost on our minds and in our nightmares that we've created an elaborate metaphorical architecture so we can talk about it even with people who are too scared to talk about it. We think about it so much, it seems almost ridiculous that we aren't ready. But we're not. Not even close.

Just look what happened with Ebola.

III. The "luck" of the Ebola outbreak

Ron Klain was an odd choice for Ebola czar.

Klain entered the Obama administration as Vice President Joe Biden's chief of staff. This was, itself, notable: Klain was chief of staff to Vice President Al Gore, too, making him the only person to serve in that position for two different vice presidents.

He quickly proved himself an exceptional fixer for the Obama administration, with a mix of policy, political, and bureaucratic chops that everyone agreed was rare. And so when President Obama needed someone to coordinate the US government's response, he turned to Klain. And Klain did his job. After a few early, botched cases, the Ebola outbreak ended on American soil. Ebola became what Americans were used to it being: someone else's problem.

But talk to Klain today, and he doesn't sound like a guy exulting in victory. He sounds scared. He doesn't think Ebola showed that America's response can work. He thinks it showed how easily it could fail.

"You can’t use the word lucky or fortunate about something like Ebola that killed 10,000 people," Klain says. "But it was the most favorable scenario for the world to face one of these things. Ebola is very difficult to transmit. Everyone who is contagious has a visible symptom. It broke out in three relatively small countries that don’t send many travelers to the US. And those three countries have good relationships with America and were welcoming of Western aid."

"With a pandemic flu, the disease would be much more contagious than Ebola," Klain continues. "The people who are contagious may not have visible symptoms. It could break out in a highly populous country that sends thousands of travelers a day to the US. It could be a country with megacities with tens of millions of people. And it could be a country where sending in the 101st Airborne isn’t possible."

Ebola, Klain thinks, shows how unprepared the world was for a disease that it's known about for decades and that, comparatively speaking, spreads pretty slowly. A person infected with Ebola can be expected to pass the disease on to two people, barring effective countermeasures (epidemiologists call this the "reproduction number"). Two is not that high, as these things go. The SARS virus had a reproduction number of four. Measles has a reproduction number of 18.

What happens when the world faces a lethal disease we're not used to, with a reproduction number of five or eight or 10? What if it starts in a megacity? What if, unlike Ebola, it's contagious before the patient is showing obvious symptoms?

Past experience isn't comforting. "If you look at the H1N1 flu in 2009," Klain says, "it had spread around the world before we even knew it existed."

IV. How human beings have helped infectious disease

Behind Gates's fear of pandemic disease is an algorithmic model of how disease moves through the modern world. He funded that model to help with his foundation's work eradicating polio. But then he used it to look into how a disease that acted like the Spanish flu of 1918 would work in today's world.

The results were shocking, even to Gates. "Within 60 days it's basically in all urban centers around the entire globe," he says. "That didn't happen with the Spanish flu."

The basic reason the disease could spread so fast is that human beings now move around so fast. Gates's modelers found that about 50 times more people cross borders today than did so in 1918. And any new disease will cross those borders with them — and will do it before we necessarily even know there is a new disease. Remember what Ron Klain said: "If you look at the H1N1 flu in 2009, it had spread around the world before we even knew it existed."

Gates's model showed that a Spanish flu–like disease unleashed on the modern world would kill more than 33 million people in 250 days.

"We've created, in terms of spread, the most dangerous environment that we've ever had in the history of mankind," Gates says.

V. Underdeveloped health systems threaten developed countries

The science fiction writer William Gibson has a good line: the future is already here, it's just not evenly distributed. And nowhere is that truer than in health care.

According to the World Health Organization, the United States spends more than $8,000 per person, per year, on health care. Eritrea spends less than $20. Traditionally, Americans thinks of that as Eritrea's problem. But if a highly infectious, highly lethal new disease presents in Eritrea, and the world is slow to learn about it, then it will quickly become America's problem.

This is, of course, what happened with Ebola. If it had made its first appearance in the United States, it likely would have been caught, and contained, quickly. But as my colleague Julia Belluz wrote, the countries where the 2014 outbreak began "happen to be three of the poorest in the world, and it took them at least three months to even realize they were harboring an Ebola outbreak." By the time Ebola was recognized, it was already out of control — and so, for the first time, it made its way to American shores.

When I ask the CDC's Frieden what's needed to catch these diseases early, he doesn't hesitate. "The most effective way to protect people is basic public health infrastructure," he says. "That means laboratories for finding specimens, getting them tested, and discovering what's spreading. It means field epidemiologists. It means emergency operation centers. And you need to have that available day in and day out. If we've learned anything, it’s that you want an everyday public health system you can scale up for an emergency, not a system you only use in case of emergencies."

The good news is this kind of system isn't all that expensive. Basic public health infrastructure is fairly cheap — around a dollar per person, per year. "There’s no magic here," says Frieden. "In Uganda, you have motorcycle couriers picking up specimens from hundreds and hundreds of health-care centers all over the country. They then send them to centralized centers. The expense isn't huge."

The difficulty often isn't money; it's priorities. These aren't sexy investments. "It doesn’t cost nearly as much as building a fancy hospital in your capital," says Frieden, with evident frustration.

But if you can find the disease and test it, then modern technology really does come into play. We can rapidly decode the basic structure and pathways of new diseases in ways that were unimaginable even a few decades ago — and that means we can come up with a response much more rapidly.

That's the good news.

The bad news? "You need a government that works," sighs Frieden.

VI. "Are we sure [the WHO] can do better next time? No."

Pandemic infections present three basic problems of governance. The first is countries that don't want to admit they need international help because they don't want to admit they have a problem in the first place.

"Guinea did not want to declare an Ebola epidemic," Gates says, "because in terms of investors and travel, it's a death sentence."

And it wasn't just Guinea, or even just Ebola. As Michael Specter wrote in the New Yorker:

If SARS had been more contagious, it would have created the new millennium’s first grave public-health crisis. And yet, in 2002, after it first appeared, Chinese leaders, worried about trade and tourism, lied about the presence of the virus for months—insuring that it would spread. In 2004, when avian influenza first surfaced in Thailand, officials there displayed a similar reluctance to release information.

The second is countries that can't admit international help, either because the state is too weak and fragmented to effectively coordinate with international actors or because the state is hostile to the organizations that would need to come in and offer relief. Imagine an outbreak that begins in Syria right now, and you get the idea.

The third problem is that no one really trusts the efficacy of the international institutions that would most naturally coordinate the response.

There is no other way to say this: the World Health Organization's Ebola performance was a disaster. "The WHO’s slow response to Ebola has been universally condemned," reported the Guardian. "The director general’s committee — which can declare a public health emergency — was not convened until August, eight months after the first cases and five months after public warnings from Médecins Sans Frontières, whose doctors were on the front line."

Germany's Chancellor Angela Merkel is now leading an effort to reform the organization. But similar mistakes during the SARS crisis and H1N1 have led to similar calls for WHO reform, and little has happened.

This isn't just an issue of bureaucratic incompetence. The WHO is underpowered for the problems it's meant to solve. About 75 percent of its funding comes from voluntary donations, and there's no mechanism by which it can quickly scale its efforts during an emergency. The WHO's member countries could fix this by giving the WHO more reliable, permanent funding — or even more reliable emergency funding mechanisms. But so far, no suggestions along those lines have gained much traction.

The result is that the WHO that will face the next major disease outbreak is likely to be quite similar to the WHO that faced Ebola, and H1N1, and SARS. As a senior US delegate to the World Health Assembly told Vox, "Are we sure [the WHO] can do better next time? No."

Whether through the WHO or some other mechanism, most experts agree that the world needs some kind of emergency-response team for dangerous diseases. But no one knows quite how to set up that team. "That's what we’re lacking in the global system — a battalion of people in white helmets," says Klain. "But who will own it? Control it? Pay for it? Deploy it? Those are the tricky things."

This is in stark contrast to war, which is not necessarily more deadly to the human race, but is much better planned for. "When you talk about war," Gates says, "there are all these rules about how the government can seize various ships. But when an epidemic comes along, who is supposed to survey the private capacity and go out there and grab all these things?"

Look at what happened during Ebola, Gates continues. "Where was the equivalent of the military reserve, where you get on the phone and you said to people, Now come! And they had been trained, and they understood how to work together. People who want to volunteer, do we pay them? What do we do with them after they come back, when people might have this fear that they've been exposed? Are employers going to take them back? What are the quarantine rules? It was completely ad hoc."

This is what's so maddening about the modern fight with epidemic disease. Unlike in past eras, humanity has the tools it needs to protect itself. But global travel has far outpaced global governance — or even global disease response. Diseases move much faster than governments. "This is the hole in the global system," Klain says, and no one really knows how to fix it.