In his first interview since his weeklong stay in a hospital's intensive care unit, Christie said he's in recovery but "not yet 100%." He said COVID-19, which has killed more than 217,000 Americans, came on "like a freight train."
"All of a sudden I got a fever, chills, and body aches, and I was wracked with pain and exhaustion," Christie said.
Christie had spent four days with Trump on the White House grounds preparing for his first debate against former Vice President Joe Biden. He also attended the "superspreader" event following the nomination of Amy Coney Barrett to the Supreme Court.
"It was a mistake," he said. "I was led to believe that all the people I was interacting with at the White House had been tested, and it gave you a false sense of security."
Christie told "GMA's" George Stephanopolous that he had been vigilant about mask wearing, social distancing, and hand washing for seven months prior to his diagnosis, but he let his guard down.
"I let my guard down and it was wrong it was just a big mistake," he said. "I was doing it right for seven months. I let my guard down for a couple of days inside the White House grounds and it cost me in a significant way. I was wrong."
Christie did not condemn the president, who has been an inconsistent advocate for mask wearing, holding large rallies of thousands of people where many of his supporters do not follow public health guidance to cover their faces.
But the former governor said he thinks all public officials should be more proactive in encouraging face coverings.
"There is no downside to you wearing masks, in fact, there's an upside," he said. "We don't know everything we need to know about this disease, but we do know that masks can certainly help."
At a town hall in Miami on Thursday night, Trump said of Christie's statement, "He has to say that." The president insisted he supports masks and called Christie "a friend of mine."
The Associated Press contributed to this report.