Trump has begun quarantine after exposure to COVID-positive Hope Hicks -- and is off the campaign trail

President Donald Trump is required to quarantine for 14 days after flying four times in the last 72 hours with Hope Hicks, who tested positive for COVID-19 on Thursday.

Dr. Ashish Jha, the Dean of the Brown University School of Public Health, explained the implications of Hicks' test during an appearance on MSNBC's "The Last Word" with Lawrence O'Donnelll.

"It is a real risk for the president," Dr. Jha explained. "First of all, I still can't believe that the White House staff don't wear a mask around the president, that the president doesn't wear a mask. I appreciate the fact that they're getting tested all the time, that is not enough, masks are still essential."

He explained why guidance from the Centers for Disease Control require Trump be quarantined for two weeks.

"And at this point, the CDC guidance on this is very clear: all of the people who have been around her over the last few days should self-quarantine, for 14 days, and that includes the president," he noted.

The interview occurred shortly after Trump revealed on Fox News that he has been tested again, but appeared unfamiliar with the quarantine guidance from the CDC.

“So whether we quarantine or whether we have it, I don’t know,” Trump told Sean Hannity. “I just went out for a test — they just do it, it will come back later I guess — and the first lady also because we spend a lot of time with Hope, and others, so we’ll see what happens."

Shortly after the clip aired, Trump addressed the quarantine question on Twitter.

Watch: