(CNN) -- In an exclusive interview with CNN, Dr. Thomas Frieden, the director of the Centers for Disease Control and Prevention, reflected on the lowest moments -- and the surprises -- that occurred when Thomas Eric Duncan became the first person to unexpectedly arrive in the United States with Ebola.
Duncan has been the only person in the United States to die of the disease, and two nurses who cared for him became infected with the virus. The CDC was roundly criticized for its handling of the crisis, and some Republican lawmakers called for Frieden's resignation.
Frieden's lowest moment: when the first nurse, Nina Pham, was diagnosed with Ebola.
'We learned that the situation wasn't going to be as simple or as controlled as we thought it would be, and we immediately intensified our response to address the situation in Dallas,' he said.
He said his biggest surprise was the difficulty of Duncan's care.
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'I think we didn't recognize how hard it would be to care for someone with Ebola who was desperately ill in the U.S., and how much hands-on nursing care there would be, and we didn't expect two nurses to get infected,' Frieden said.
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Ebola victim's nephew: We make mistakesDuring Duncan's hospitalization, Frieden repeatedly said in press conferences that any hospital in the United States should be able to treat Ebola.
CNN asked Frieden if he regretted those statements, considering that at Texas Presbyterian Hospital, Duncan died and two nurses became infected, but the other nine U.S. Ebola patients, who were treated at hospitals that were specially designated to treat the disease, all lived. In those cases no workers became infected.
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'Clearly there are things that we wish we'd done differently,' he said. 'The bottom line is that Ebola is hard to treat, and when the first patient ever with Ebola came to the United States, we thought the guidelines would protect the health care workers. When two health care workers became infected, we recognized the guidelines didn't work. So we changed them.'
He added that one thing his agency would have done differently would have been to tell nurse Amber Vinson not to board an airplane from Cleveland to Dallas. Vinson had taken care of Duncan and had reported a temperature of 99.5 degrees, but was told she could fly, because the CDC's threshold at that time for Ebola was 100.4 degrees. Vinson later was diagnosed with Ebola.
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CNN also asked if he considered leaving his post when several Republican lawmakers called for his resignation.
'I'm really just focused on stopping Ebola. I'm working around the clock to do that, and I will continue to do that, as long as Ebola is spreading, as long as I have the opportunity to do that,' he said.
'I wish I had know then what we know now,' he added. 'But that's not how the world works. We live life forwards and we understand it backwards. Looking back with 20/20 hindsight, there are always things we would do differently.'
CNN's John Bonifield contributed to this story.