New Video: Danny Trejo Throws Punch and Chair After Water Balloon Attack

JOE BIDENI WAS SO SICK I TOOK A COVID TEST!!Disastrous Debate Tell-All

Joe Biden felt so under the weather going into the Presidential debate with Donald Trump that he took a COVID test -- this he tells ABC News' George Stephanopoulos.

Check out this preview clip from the full ABC special airing later Friday -- Stephanopoulos grills Biden on whether his rough performance during the June 27 debate, which left Americans worried about his ability to lead, was just a bad episode or a sign of something more serious.

Joe insists it was simply a one-off, saying, "I was exhausted. I didn't listen to my instincts in terms of preparing and -- and a bad night."

GS points out Biden spent nearly a week at Camp David prepping for the debate ... and asks why that wasn't enough time for him to get ready.

Biden tells him it was all down to him feeling sick -- something we knew before, but he details just how terrible he was feeling, adding, "I asked if they [the docs] did a COVID test because they were trying to figure out what was wrong."

He explains, "They did a test to see whether or not I had some infection, you know, a virus. I didn't. I just had a really bad cold."

Biden also blasts his opponent Trump's performance during the interview, calling out the numerous false statements he made. But when it comes to his side of things, seems like Biden's taking responsibility, saying it was no one's fault but his own.

As you know ... many Democratic donors and party leaders questioned Joe's mental health following the debate, where he suffered numerous lapses.

Some even demanded he immediately bow out of the presidential race -- but Joe hasn't budged an inch and is forging ahead with his campaign.