There is an animal somewhere in Africa — most likely a bat — that has worked out an arrangement with a microscopic agent. The deal is this: the agent won’t kill the bat if the bat will transport it to other warm-blooded animals and give it a chance do its gruesome work. All the bat had to do to enter this arrangement was build up a resistance to the agent over generations and become a good hiding place — and then continue about its business of being a bat.

Long before they provided cover for vampires, bats were reservoirs for viruses.

We identified such an agent in 1976 and named it Ebola for a nearby river. Unfortunately, we didn’t find it in a bat but as a virus in the blood of a dead man.

A virus that kills quickly does not take full advantage of the social behavior of humans and tends to burn itself out. That behavior includes the profound compassion of health care workers who are always among the secondary infections; funereal practices that bring the healthy in contact with the infected dead; and illiteracy, which keeps the local population from understanding what is afoot. The very lethality of Ebola — killing up to 90% of its victims — becomes a self-limiting proposition. It will never become a pandemic, according to public-health experts, unless we help it along.

And how would we do that?

Public health is a kind of math class we seem to fail year after year. Its most basic equation addresses the following question: for every infected person today, how many more infected people can we anticipate? The numerical answer to this question is called the R-nought of the disease. Smallpox has an R-nought of between 3 and 7, depending on population density. The Spanish flu of 1918 had an R-nought between 3 and 4 and killed an estimated 100 million people. Ebola has an R-nought of 1.5.

The people who are infected with Ebola develop a screenwriter’s list of symptoms: bleeding from the mouth, nail beds and eyes as their capillaries disintegrate inside them. Their brains, awash in the blood of hemorrhagic fever, become deranged. There is no vaccine and there is no cure approved for use.

It is a terrifying prospect.

And there is no more effective contagion than fear. Rest assured, it has an R-nought far greater than Ebola. To contract it you do not need to have contact with bodily fluids, only limited exposure to sensationalizing media or a water-cooler conversation embellished with misinformation. And fear has a tendency to shut down the parts of our brain we need most in these moments and leave us at the mercy of our most primitive urges.

There is an equation used in the security world that would help inoculate us against the paralysis and bad judgment symptomatic of fear. It goes like this: risk = threat x vulnerability x consequences. In the case of Ebola, the threat is isolated to West Africa. If you have not traveled to any of the countries involved, your level of threat is zero. Even if you have visited these countries, you would still need direct contact with a sick person or animal — or the American doctor or missionary being treated in isolation at Emory University Hospital in Atlanta. But they are isolated and being treated by people who understand the equation above. Furthermore, your vulnerability is next to nil given our relatively robust public-health system that protects us from such an outbreak and, given the advanced medicine that exists in the U.S., even the consequences of such an infection are much lower.

Contrast this with places like Sierra Leone, Liberia and Guinea. The threat is clear and present, and there couldn’t be a more vulnerable population. These are countries struggling to emerge from years of civil war and violence, poor places with little to spend on public health. Pulitzer Prize–winning journalist Laurie Garrett has pointed out that Liberia spends $18 per capita on public health, Sierra Leone spends $13 per capita and Guinea a mere $7 per capita on the health of their people. (By contrast Hawaii spends $155 per capita on public health.) In addition, their cultural practices and distrust of outside aid make the consequences that much more dire. The death toll from the current Ebola outbreak tops 800. Yet 1.5 million people will die of malaria this year without the proportional coverage to the threat it poses, many of them dying in the same cash-strapped hospitals treating the current victims of Ebola.

So what should we be afraid of?

On the heels of 9/11, five deadly cases of anthrax shut down the government. And yet when 200,000 died from last year’s influenza, less than 37% of the population opted for a flu shot. It is our inability to assess risk that should scare us into action. The threat of influenza is high: we are all vulnerable regardless of geography, and the consequences can be extreme. The notion that vaccines can cause autism has long been discredited, but many of us still suffer from this fear that prevents us from protecting ourselves, our children and our neighbors.

The monster we can see — the nuclear bomb, the fanatic with the suicide vest, the swirl of hurricane in the satellite photo — leads us to build shelters, change security policy or head for high ground. But the monster in the microscope seems to sneak up on us every time. There is, without a doubt, another bat in another tree harboring another agent. But maybe this bat is in Southeast Asia or South America or in another war-torn country that can’t provide medical care for its people. And there are migratory birds crisscrossing our borders and differing standards of health care that are consorting with livestock and bringing with them novel viruses that will play genetic roulette with our collective futures. These are the real risks. This is the math exam the future holds for us.

The author would like to thank Dr. Larry Brilliant, president of the Skoll Global Threats Fund, and Dr. Alex Garza, former assistant secretary and chief medical officer of the U.S. Department of Homeland Security, for their guidance on this piece.

Burns is a screenwriter, director, producer and playwright. He wrote the screenplay for Contagion, directed by Steven Soderbergh, and produced the Academy Award–winning documentary An Inconvenient Truth.

Contact us at letters@time.com.