Dr. Anthony Fauci on Sunday spoke about President Biden’s age following the president’s much-criticized debate performance against former President Trump.

Fauci, the former director of the National Institute of Allergy and Infectious Diseases, appeared on ABC's "This Week" and was asked about whether Biden, who is 81 years old, should be running for re-election.

"You know, I think it's just an individual choice, and you really can't generalize," Fauci said. 

Fauci, who helped lead the nation’s response to the coronavirus pandemic, said one must look at each person as an individual and evaluate "how they feel, what they feel they can do."

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Anthony Fauci

Fauci said his dealings with Biden were "very positive." (Saul Loeb/AFP via Getty Images, File)

"You know, what their passion is, what their energy is," Fauci said. "Those are the kind of things."

When asked if he was surprised by anything he saw during Thursday’s presidential debate, Fauci said he did not want to comment on anything that "would have political implications." 

Biden on debate stage

Biden's debate performance was largely criticized by the media. (Eva Marie Uzcategui/Bloomberg via Getty Images)

He added that what he felt comfortable speaking about were his "very positive" dealings with Biden during their time working together.

"He asks probing questions. He's right on point on things," Fauci said of Biden, without going into detail on specific instances. "So my personal experience has been quite positive with him."

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With a raspy voice and delivering rambling answers, Biden struggled during portions of Thursday night's debate. He also lost his train of thought several times, raising concerns among some Democrats and in the media. 

In the wake of the debate, both The New York Times and Atlanta Journal-Constitution editorial boards have called on Biden to step down as the Democratic nominee.

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The Democratic National Committee, however, has said it will continue to support Biden as the nominee.

Fox News’ Sarah Rumpf-Whitten contributed to this report.