Photograph by Samuel Aranda/The New York Times/Redux.

Since September 11, 2001, Americans have had to adjust to an unsettling truth: as the world becomes smaller and easier to traverse, it also becomes more dangerous and difficult to control. There are simply not enough fingers to put in all the dikes. So we do our best: At airport security, we take off our shoes and dutifully toss water bottles into the trash. On buses and in the subway, we are often reminded, sensibly enough, that if we see something we should say something.

But as the world’s worst Ebola epidemic yet spreads through western Africa, it is important to remember that we won’t always see something. “The single biggest threat to man’s continued dominance on this planet is the virus,” the Nobel Prize-winning biologist Joshua Lederberg once wrote. Few epidemiologists would disagree. There is no bomb, no poison, no plan of attack with the potential to do as much damage.

It doesn’t take much effort to see that. Smallpox killed up to half a billion people in the twentieth century alone, before its eradication, in the nineteen-seventies. (That’s why it was so terrifying to learn, last month, that vials full of smallpox, alive and forgotten, had been lying for decades in the refrigerators of a former N.I.H. laboratory.) The global public-health system needs to become far more vigilant in detecting new viruses before they spread. That will require patience, time, and money—an unlikely combination at best.

On Thursday morning, the President of Sierra Leone cancelled a planned visit to the United States, declared a national health emergency, and ordered the Army to quarantine people in the worst-affected areas. The Liberian government has shuttered the country’s schools and placed most public employees on a thirty-day leave. The Peace Corps this week pulled three hundred and forty volunteers out of Liberia, Guinea, and Sierra Leone.

As many as ninety per cent of those infected with Ebola will die. There is no cure or treatment. There are several vaccines under development; in early animal tests, more than one has shown promise. But it will be years before they are ready for humans. Until then, if you get Ebola, you are most likely done for. The virus can eat away at capillaries and blood vessels, causing you to drown in your own blood. As David Quammen wrote in “Spillover,” the definitive book about the origin and evolution of human epidemics, “Advisory: If your husband catches an Ebola virus, give him food and water and love and maybe prayers but keep your distance, wait patiently, hope for the best—and, if he dies, don’t clean out his bowels by hand. Better to step back, blow a kiss, and burn the hut.”

Still, Ebola’s more prosaic symptoms—abdominal and muscle pain, fever, headache, sore throat, nausea, and vomiting—also apply to at least a dozen other conditions. Could an infected airline passenger make it to the United States? Absolutely. But in this country every doctor and nurse in every clinic and hospital uses gowns, latex gloves, masks, and disinfectants. Those precautions are rarely available in the parts of Africa where the epidemic has been most severe. Ebola is contagious only when it is symptomatic, and by that time people are almost invariably too sick to travel. (Patrick Sawyer, the only American to die so far in this outbreak, collapsed after a flight from Liberia to Lagos. He was planning to fly next to Minnesota. He never got on that plane.)

“I wouldn’t be worried to sit next to someone with the Ebola virus on the Tube, as long as they don’t vomit on you or something,” Peter Piot told Agence France-Presse this week. Piot, the director of the London School of Hygiene and Tropical Medicine, was one of the two people who, in 1976, discovered Ebola. He then ran the United Nations' AIDS program for more than a decade. “This is an infection that requires very close contact,” he said.

Ebola is truly deadly, but the many lurid headlines predicting a global pandemic miss a central point. In its epidemic reach, Ebola is often compared with H.I.V. But they are nothing alike. H.I.V. has killed at least thirty million people, mostly by spreading quietly, burrowing into the cells it infects, and then, at times, lurking for years before destroying the immune system of its host. Ebola’s incubation period is between two and twenty-one days long. The virus kills rapidly. There is nothing insidious about it.

Ebola won’t kill us all, but something else might. Like everything living on Earth, viruses must evolve to survive. That is why avian influenza has provoked so much anxiety; it has not yet mutated into an infection that can spread easily. Maybe it never will, but it could happen tomorrow. A pandemic is like an earthquake that we expect but cannot quite predict. As Quammen puts it, every emerging virus “is like a sweepstakes ticket, bought by the pathogen, for the prize of a new and more grandiose existence. It’s a long-shot chance to transcend the dead end. To go where it hasn’t gone and be what it hasn’t been. Sometimes the bettor wins big.”

He’s right, of course, and it is long past time to develop a system that can easily monitor that process. If we don’t, the next pandemic could make Ebola look weak.