Ebola preparation efforts in New York City Photo: Spencer Platt/Getty Images

At noon, CDC Director Thomas Freiden is appearing before the House Energy and Commerce Committee, for what will surely be an aggressive interrogation over the government’s response to the Ebola crisis. The rhythm of the public debate over the domestic contagion so far has been that of two knees jerking, one right after the other — first lower-brow media and Republicans, going hysterical about the prospect that the contagion might spread, and then smarter media and Democrats, going hysterical about the hysterics. Anticipating the show, you feel a twinge of sympathy for Freiden, who is bound to endure the least fun or enlightening part of the political circus, in which every politician and commentator assumes a position of extreme certainty about a topic they know nothing about. And yet it is hard to avoid the sense that Freiden himself has contributed to this atmosphere. The Times’ story previewing the hearing elicited a telling quote from Michael Osterholm, who runs the Center for Infectious Disease Research and Policy at the University of Minnesota. “We’ve got to stop being so certain.”

For me, this has been one of the major themes that has run through the whole Ebola episode, from its initial outbreak in Guinea and Sierra Leone to the horror stories from the quarantine zones in Liberia to the tentative victories in Nigeria and Senegal to the breach of the American border: We have been a little too certain. Not too slow to take the contagion seriously, nor too cautious about the likelihood that the virus would spread — public-health officials and even most of the media seemed to get that right, all along — but more simply too sure that we understood the exact dimensions of the virus, and the conditions under which it would or would not spread. “We will stop Ebola in its tracks,” Freiden has said, again and again, and though these were reassuring statements, they also may have given some false confidence.

The problems of certainty in the face of a deadly virus have been apparent over the past few days, with the news that two nurses at Presbyterian Hospital in Dallas (Nina Pham and Amber Vinson) contracted the virus while caring for a Liberian patient with Ebola, Thomas Duncan. At each step there were errors to be regretted. Duncan was sent home when he was first admitted to the ER, even though he told a nurse interviewing him that he had recently arrived from the Ebola zone. He was cared for at his local hospital, rather than being transferred to one of the four major centers for Ebola response. Within Presbyterian Hospital there was a great deal of confusion about exactly how to use the protective gear that they had been given to help keep the virus from spreading; the natural conclusion, though of course we don’t know, is that this may have contributed to two nurses acquiring the virus. Vinson flew to Cleveland and back, even though she had been told not to, and a CDC officer who interviewed her before she got on her flight out of Dallas allowed her to go because her temperature was not very elevated — a test that is being applied even though there is scant reason to think temperature screening is effective. Vinson “should not have traveled,” Freiden said, but if the CDC were less certain that the system would work then it could have explicitly banned her. “It’s not that challenging,” an exasperated Sanjay Gupta said on CNN yesterday, blaming the medical staff at Presbyterian Hospital for the spread of the virus. “We’re talking about covering your skin. Cover your skin!”

In a theoretical case, in a laboratory environment, perhaps it is as simple as that. (Though a little easier to screw up than you might think.) But this is the first time that the global health system has seen Ebola on this scale, and the first time Western health systems have seen it at all. Caring for people with Ebola in a modern health-care system, in other words, is a novel human experience, and it is subject to human uncertainty and error. The key question to ask turns out not to be whether the CDC can say what procedures nurses need to take to keep themselves from contracting the virus, but whether nurses with minimal special training will actually be able to take them, and whether a hospital that the virus chose to visit at random will be able to concisely explain it to them. The key question turns out not to be whether nurses will ask a new febrile patient in the ER whether he has traveled to West Africa recently, but whether — under the routine stress of life in a hospital — nurses and doctors will be able to accurately weigh the importance of that information. Human systems like this are more complicated than they look.

The smart media has, for weeks, looked at this virus and reminded audiences of the comparatively low numbers of the infected and dead, compared to — for instance — the flu. We have looked with some horror at the inhumane conditions in which medical workers have had to labor in Sierra Leone and Liberia, deprived of basic equipment like hazmat suits and cleaning solutions, and suggested that the real story of Ebola was a resource story, and that in a country that had not only plenty of hazmat suits but also the Centers for Disease Control and the Massachusetts General Hospital and the best medical minds on the planet, the experience of the disease would be vastly different. Which, in the medium-term, will almost certainly be true. But in the first experience of a disease all of these great systems are pressured in ways that are hard to anticipate — the chance that the virus would first breach the United States in the Presbyterian Hospital ER was virtually zero, and yet it did. A contagion like this, as Freiden will likely be reminded today, begs for a little more human humility.