Congolese men get smallpox vaccinations at a clinic in 1962. The infection was officially declared eradicated in 1980. (Associated Press)

Committing blunders doesn’t mean you’re incompetent. Not being able to explain how something happened doesn’t mean you’re clueless. Yet we think public health and medicine should somehow escape those truths.

The experience of Ebola in the United States can be summarized as this:

No sooner had the virus arrived on American shores than, wormlike, it had found all the gaps in American health care, epidemic preparedness and public health authority. The hospital in Dallas where the first imported case presented was asleep at the switch; the nurses who cared for the tardily diagnosed patient were sloppy; the federal agency that might have prevented their sloppiness failed to give them the advice they needed. Guidelines for how to protect health-care workers and isolate potential patients changed on the fly. Reassurance from the Centers for Disease Control and Prevention rang hollow.

Did anyone know what they were doing? That’s a question columnists, members of Congress and a lot of ordinary citizens asked. It’s also a reasonable question, and one that history can help answer.

Take the example of smallpox eradication.

New Yorkers stand in line, waiting to be inoculated against smallpox at the Morrisania Hospital in the Bronx on April 14, 1947. (Associated Press)

Smallpox is the only human infection man has gotten rid of; its official eradication in 1980 was one of the great medical achievements of the 20th century. The 10-year campaign to finally kill it off, however, ended in a symphony of screw-ups.

Epidemiologists working under the auspices of the World Health Organization 37 years ago believed they had found the world’s last cases of the viral infection in a band of herdsmen in southern Somalia.

On Oct. 12, 1977, a Land Rover carrying two patients, one a critically ill 6-year-old girl, drove 50 miles from a nomad encampment to Merca, a city on the Indian Ocean. The vehicle stopped at a hospital there to get directions to the smallpox isolation camp outside town. A 23-year-old cook agreed to direct the driver, and he got into the car. The ride took five minutes.

The cook, Ali Maow Maalin, had worked as a smallpox vaccinator, but it turns out he’d never been vaccinated himself. Two weeks later, he developed a fever and went to the hospital. The doctors thought he had malaria. When he developed a bumpy rash, they concluded he had chickenpox and sent him home. Maalin, however, soon figured out his problem. But he didn’t want to go into isolation, so he didn’t notify authorities. An acquaintance turned him in.

Nobody could believe Maalin had smallpox. But he did—and he’d had contact with at least 100 people while he had it. Officials closed the hospital, vaccinated the occupants of the 800 houses in Maalin’s neighborhood and eventually set up checkpoints on the one road and three footpaths leading to town. To enter or leave Merca, you had to be vaccinated. Over two weeks, 54,777 people were inoculated.

And that was the end of smallpox.

The smallpox lesson

The eradication campaign took a decade of skill, investment, dedication, patience, courage and luck. But it wasn’t only that. In a 2009 book, the person who led the campaign, American physician D.A. Henderson, wrote that Maalin’s case “was a classic one in depicting omissions and mistakes in program operations.”

The ruins of the Smallpox Hospital stand near the Franklin D. Roosevelt Memorial Park on Roosevelt island in New York. (Mary Altaffer/Associated Press)

Henderson’s bland description of the tragicomic endgame may have served to protect WHO’s rear end three decades later. But it also acknowledged the obvious: “Omissions and mistakes” happen. As the country enters the Ebola era, it’s useful to ask: Why is that so?

The first and biggest reason is that medicine is conservative. It assumes things behave the way they should and usually do. Vaccinators are vaccinated, five minutes’ exposure doesn’t get you infected, diseases don’t change their symptoms, treatments don’t stop working.

Of course, germs can develop resistance to drugs, ailments such as asthma can become more prevalent, new pathogens like the Ebola virus can appear out of nowhere.

But medicine would become an unmanageable task if physicians were constantly acting as if the rules didn’t hold and that every new case inhabited one of the tails of the bell curve (where rare events live) rather than the curve’s fat middle part (where common ones do).

The trick for doctors is to keep both possibilities in mind. Early in their training, medical students learn an aphorism that comes in two versions with opposing meanings, both true: “When you hear hoofbeats, don’t think of zebras” and “When you hear hoofbeats, don’t forget about zebras.” In other words, expect the common, but don’t rule out the unusual.

When the Centers for Disease Control and Prevention’s director, Thomas Frieden, failed to send a team to Texas Health Presbyterian Hospital in Dallas to oversee infection control as soon as Thomas Eric Duncan, a Liberian visitor, was diagnosed with Ebola, he was assuming that doctors and nurses who take care of highly infectious patients in a 900-bed hospital would be able to protect themselves.

He was wrong.

It turns out that the usual protective measures weren’t enough. Why isn’t clear. Many unprotected people in close contact with symptomatic patients don’t become infected. (That now includes all the people who shared living quarters in Dallas with Duncan and were released last week from a 21-day isolation.) That two protected health-care workers would immediately become infected — and without an obvious breach in practice — was a surprise. Should CDC have anticipated it? Maybe. But the fact that it didn’t doesn’t suggest incompetence.

Anthrax experience

CDC officials made a similar wrong assumption, which proved fatal, in the anthrax letter attacks of 2001. They failed to recommend that hundreds of postal workers at the Brentwood mail-sorting center in Washington take antibiotics, assuming incorrectly that anthrax spores couldn’t leak out of a sealed letter. Two workers died. Afterward, Jeffrey P. Koplan, the CDC director then, said: “I think people are somewhat surprised that we’re learning things on a day-by-day basis, but that’s really no different from any other investigation that we’ve done this year, five years [ago] or over the last 50 years.”

That may sound like an excuse, but even in life-and-death circumstances, learning is based on experience.

The only egregious mistake in the Ebola outbreak so far was the failure of the doctors treating Duncan in his first visit to the hospital emergency room to learn that he’d just come from West Africa. Taking a “travel history” is the responsibility of every physician examining someone with a fever. That’s drilled into medical students. Failure to ask, “Have you traveled recently?” is a failure of tradecraft; it doesn’t matter that the triage nurse got the answer and the electronic medical record failed to pass it along.

The public also needs to become tolerant of unexplained events and contradictory actions — at least up to a point.

How did the two nurses get infected? We’ll probably never know, just as we still don’t know how Kathy T. Nguyen, a 61-year-old New Yorker who worked in a hospital stockroom, contracted fatal anthrax during the 2001 attacks. She may have inhaled bacteria from a “cross-contaminated” piece of mail. But that’s just a guess. Low-probability events are, by definition, hard to explain.

Demands from legislators that begin with “Can you guarantee me . . . ” are never going to be answered with an unequivocal “Yes.”

Public health is a dull sword, but it is a strong one. It doesn’t cut finely, and nobody should expect it to. But when wielded by people willing to learn from their mistakes, it tends to get the job done.

Just ask the smallpox virus.

Brown, a physician, covered science and medicine at The Post for 22 years.