US President Donald Trump has been reunited with first lady Melania at the White House after three nights being treated for COVID-19 at a military hospital outside Washington.

Wearing what appeared to be a white surgical mask, Mr Trump, 74, pumped his fist and gave a thumbs up as he walked down the steps of the Walter Reed Medical Centre at Bethesda, Maryland, on Tuesday morning (Australian time).

He responded to a reporter’s question about how many people were infected at the White House by saying: “Thank you very much.”

The Republican president, who is running for re-election against Democrat Joe Biden on November 3, was admitted to the hospital on Friday after being diagnosed with the disease caused by the novel coronavirus.

Mr Trump was driven a short distance from the hospital to his waiting helicopter and flown across Washington to the White House. In a show of strength, he disembarked, walked up the front steps of the White House without assistance, and stood resolutely.

He then saluted the helicopter as it took off and, within the hour, took to social media to urge Americans not to be afraid of COVID-19.

“I learned so much about coronavirus and one thing that is for certain, don’t let it dominate you, don’t be afraid of it, you’re going to beat it. We have the best medical equipment, the best medicines.

“We’re going back to work, we’re going to be out front, as your leader I had to do that, I knew there was danger to it, I stood out front, I led.

“Now I’m better. And maybe I’m immune, I don’t know. But don’t let it dominate your lives. Get out there, be careful, we have the best medicines in the world. And it all happened very shortly, and they’re all getting approved, and the vaccines are coming momentarily.”

Tweet from @realDonaldTrump

White House press secretary Kayleigh McEnany, who tested positive for COVID-19 alongside two deputies, took to social media as soon as Mr Trump had disembarked the helicopter.

“Love seeing our President,” she wrote.

In an earlier statement posted on Twitter, Ms McEnany said the White House medical unit did not list reporters, producers, or other members of the press as close contacts.

But sources told CNN that two of her deputies, Chad Gilmartin and Karoline Leavitt, had also contracted the coronavirus.

There are concerns others will fall ill, after it was noted Ms McEnany had removed her mask to speak to journalists at recent briefings.

In a town hall on MSNBC on Monday (local time), Democratic nominee Joe Biden said he hoped Mr Trump “would communicate the right lesson to the American people. Masks matter.”

“There’s a lot to be concerned about – 210,000 people have died,” Mr Biden said. “I hope no one walks away with a message thinking that it is not a problem. It’s a serious problem. It’s an international pandemic.”

He also argued it should be seen as a patriotic duty to wear a mask.

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Infectious diseases specialist assistant professor John Barocas from the Boston Medical Centre, told the ABC the President was “still infectious”.

“He was only diagnosed somewhere on the order of four-five days ago. We know that the period of infectiousness is up to 14 days.

“It’s crucial at this point that for the sake and safety of everyone around him that he stay isolated in the residence, does not move around the White House. And when people do come to change his sheets and bring him breakfast, that those people are well protected with personal protective equipment,” he said.

Tweet from @BuzzFeedNews

Mr Trump had preceded his departure from hospital with a barrage of tweets.

“Feeling really good!” he wrote.

“Don’t be afraid of COVID. Don’t let it dominate your life. We have developed, under the Trump administration, some really great drugs & knowledge. I feel better than I did 20 years ago!”

The President’s medical team said earlier that Mr Trump had not had a fever in more than 72 hours and his oxygen levels were normal.

They declined, however, to discuss any toll the disease could have on Mr Trump’s lungs or disclose when he last tested negative for coronavirus.

Tweet from @JoeBiden

What will the next week look like for the President?

Professor Barocas said the President’s condition would be monitored closely for the next seven days.

“We know that people that have had moderate to severe COVID, which is what he’s had by definition, can have a falling off the cliff in the
coming days, and so I would hope that his physicians are monitoring that very closely.

“His physicians have not been forthcoming with chest x-rays
and electrocardiograms. That said, the Dexamethasone is certainly indicated, for someone with moderate to severe COVID, and the Remdesivir is also indicated. The monochronal antibody that he received is experimental.

“Honestly, the President is probably one of the only people in the world at this point that has, or is one of the only people in the world that has received that combination therapy,” Professor Barocas told the ABC via video link on Tuesday.

“He’ll likely get nearly round the clock vitals monitoring. So his heart rate, his blood pressure, his oxygen saturation. Those will be done on a regular basis.

“He’ll continue to get the medication that is he’s been prescribed, as we talked about earlier. And I hope that he will continue to rest, for his own sake.

“Stay in isolation, for everyone else’s sake. And that’s what I would presume the next few days are going to look like. Hopefully not out on the campaign trail,” he said.