“I am not protecting West Africa,” Tom Frieden, pacing in his office, tells an unhappy U.S. senator on the other end of a call from Washington. “My number one responsibility is to protect Americans from threats.”

Then: “Respectfully, sir, I don’t agree with you.”

A moment later: “I hope to regain your confidence.”

When he hangs up, Frieden doesn’t identify the senator, other than to say that he was a Republican who wants an absolute travel ban on people from West Africa because of the Ebola epidemic. Frieden thinks that’s a misguided idea that will backfire, but the senator would not be convinced otherwise.

“It was pingpong ball against iron safe,” he says.

The CDC's Emergency Operations Center fields thousands of calls a month, many of them lately regarding the deadly Ebola virus. (Michel du Cille and Casey Capachi/The Washington Post)

Frieden, the 53-year-old doctor who for the past five years has served as director of the Centers for Disease Control and Prevention, is immersed in an epidemiological and political crisis. He has become the face of that crisis, and more than that, the voice. It’s a deep voice, sonorous, and he speaks slowly, deliberately, assuredly, and he declared at the end of September: “I have no doubt that we’ll stop this in its tracks in the U.S.”

But his confident statements have had to compete with the onslaught of bad news, including the infection of two health-care workers in Dallas, and he is now on the defensive.

Frieden spent much of Thursday on Capitol Hill getting grilled by lawmakers whose concerns were similar to those of the politicians who phoned him Wednesday. They asked: Why not impose a travel ban? How can you be sure someone with Ebola won’t slip into the country and trigger another scenario like the one in Dallas?

Frieden answered that a ban on travel in and out of those countries simply wouldn’t work, because people would still find a way to move across the porous West African borders and eventually make their way by air to the United States. Officials at airports wouldn’t know to check their travel history and screen them for fever, and they wouldn’t be able to track the travelers later and monitor them for Ebola symptoms.

But Frieden has failed to persuade critics, including some Democrats, who favor a travel ban.

Ebola has killed more than 4,400 people in West Africa and is out of control in Liberia, Sierra Leone and Guinea. Frieden visited West Africa in August — going into Ebola wards to see the disaster firsthand — and his dire report helped spur the Obama administration to ramp up the response to the epidemic.

In the meantime, the Ebola news in the United States has gone from bad to worse.

This is the largest outbreak of the Ebola virus in history.

The “index patient,” Thomas Eric Duncan, brought the disease into the country from Liberia in September, and the virus later infected the two health-care workers at Texas Health Presbyterian Hospital Dallas. That number may well rise; officials are monitoring scores of workers potentially exposed to Duncan, who died Oct. 8.

Critics have said that CDC guidelines for infection control are too complicated and too hard to follow. A national nurses union said workers had inadequate training and protective equipment. Frieden this week expressed regret that the CDC hadn’t done more early in the crisis to help the Dallas hospital. He also apologized for saying that a “breach in protocol” must have occurred in the treatment of Duncan; some people had interpreted that as a criticism of the hospital or of the infected nurse Nina Pham.

The CDC’s guidelines seem to be easily evaded by the virus. Most notably, on Monday the CDC was informed by health-care worker Amber Vinson that she was running a temperature of 99.5 degrees, but the CDC did not prevent her from boarding a commercial jetliner and flying from Cleveland to Dallas. Her temperature was below the threshold of 100.4 degrees that the agency has established for the screening of travelers from West Africa. The CDC guidelines are also unclear on the exposure risk for health-care workers such as Vinson and Pham who were wearing some level of protective gear.

One member of Congress, Rep. Tom Marino (R.-Pa.), has already called for Frieden’s resignation, saying, “The reports my colleagues and I have received are utterly unacceptable, and the information provided to the public has been cryptic and in some cases misleading. This has provided a false sense of security to many of our citizens. That is exactly the opposite of the CDC director’s primary responsibilities — to communicate clearly and honestly.”

But Frieden has defenders. The White House is standing by Frieden, though President Obama suggested Thursday evening that it might make sense to have one person coordinating the federal response, without using the term Ebola czar.

And many health-care experts say it is unfair to blame Frieden for the failings of an individual hospital; the CDC provides guidance and recommendations but is not a regulatory agency with authority over health facilities.

“Right now, I think Frieden is doing exactly the right thing: He’s speaking honestly, educating, informing and training all at the same time,” said David Rosner, a public health and social history professor at Columbia University. “He’s working in a very toxic political environment but is trying to be realistic and honest as well as informative.”

Michael Osterholm, an outspoken University of Minnesota epidemiologist, this week criticized the overconfidence coming from U.S. public health experts, saying they should be more up front about the unknowns and uncertainties.

“Let’s acknowledge one thing: We’re making this up as we go. And that’s not a bad thing in public health,” Osterholm said.

Many of Frieden’s predictions about the epidemic have been accurate. After he returned from West Africa, he warned that the outbreak was spiralling out of control, that stopping it would require an enormous and immediate response from the global community, and that it was inevitable that cases would surface in the United States until Ebola was stopped in West Africa.

Frieden is well-equipped for the stress that comes with his job. Before Ebola, his biggest professional challenge came in the late 1990s when he was working in India as a CDC employee assigned to a World Health Organization effort to control a tuberculosis epidemic.

“Every day I had to face the reality that if we didn’t work faster, another thousand people were going to die,” he said during an interview Wednesday.

His mother is a retired historian and lawyer, his father was a cardiologist. Their three sons, Tom, Ken and Jeffry Frieden, are all accomplished scholars who stay in close touch and are passionate New York Yankees fans.

Ken Frieden remembers his younger brother Tom sitting for hours at the bedside of his father as he suffered from Parkinson’s disease. The father would fall asleep, but Tom would stay and still be at the bedside when the father woke up, Ken said.

Tom Frieden grew up in the suburban town of Mamaroneck, N.Y., attended Mamaroneck High School, then Oberlin College and finally Columbia University, where he earned a medical degree and a master’s in public health. He met his wife at Oberlin, and they have two children; Frieden keeps no family photos in his office, which is a spartan space dominated by a large TV screen.

Before Obama appointed him in May 2009 to head the CDC, Frieden had spent seven years as commissioner of the New York City health department, one of the nation’s largest public health agencies.

Working under then-Mayor Michael R. Bloomberg (I), Frieden was the architect of some of Bloomberg’s most controversial policies, including a citywide ban on workplace smoking, including restaurants and bars. New York City also became the first place in the United States to eliminate trans fats from restaurants. Earlier, working in the health department’s tuberculosis branch, he realized that the city’s campaign against antibiotic-resistant tuberculosis would require going out and making sure that patients finished their course of medicine. He set up a program that did so, including locking up homeless people if he had to.

To stay focused, Frieden carries with him his daily schedule and to-do list on an index card. He said he learned that habit — writing down his tasks — on his first day as a doctor. For the most important ones, stay until they are done. When you feel overwhelmed, break the problem into small parts.

He is known as data-driven and gets frustrated when people don’t understand that policies need to be evidence-driven and not shaped by politics.

Frieden’s oldest brother, Jeffry, said that even in casual conversation, Tom is always asking, “How do you know that? What’s the evidence for that?”

In Frieden’s mind, the evidence clearly shows that the United States will not be safe until the Ebola epidemic is ended in West Africa. He and other opponents of a travel ban say it would hamper relief efforts, and that Ebola could become endemic in West Africa, a situation that would pose a constant threat to that continent and by extension to the rest of the world.

“Let me tell you what would happen if the virus spreads all over Africa and into Asia. That could change the entire way we handle health care here,” he said at one point Wednesday, speaking by phone to a distant lawmaker. His voice rose: “The risks to this country will be many times greater than it is today.”

There have been many such calls.

“Could we have a few more cases? Yes,” he told another senator. “Will we make sure to stop the chain of transmission? Yes.”

He added: “People are angry at me, but I’ve always taken the view that when you level with people and tell the truth, at some point, they will recognize that.”

Bernstein and Achenbach reported from Washington. Alice Crites contributed to this report.