Mr Trump's personal doctor said the president was working from the hospital where he is receiving treatment.

But what happens if a US president becomes too unwell to work?

Lawmakers have planned for this, and put it into effect in the past. The 25th amendment to the constitution places the "powers and duties" of the office into the vice-president's hands.

That would mean Vice-President Mike Pence would temporarily take charge. He has confirmed that he and his wife have tested negative for coronavirus.

Read more about exactly how this process works here.

Image source, Reuters

The president has tweeted again from hospital. Earlier he talked about "feeling well" and thanked medical staff at Walter Reed, but his second tweet was about stimulus for the US economy.

"OUR GREAT USA WANTS & NEEDS STIMULUS. WORK TOGETHER AND GET IT DONE. Thank you!" the tweet read.

Both the president's family and his personal doctor said he was working while staying at the hospital. The presidential suite at Walter Reed also has an office.

The White House says Trump's doctor misspoke when he earlier said that the president was "72 hours" into his illness.

A White House official told reporters that Dr Sean Conley had meant to say that Trump was on "Day 3" of the infection, counting from Thursday evening.

Seventy-two hours would place the president's diagnosis on Wednesday morning - which as our North America reporter Anthony Zurcher points out, is before the president travelled to Minnesota for a campaign rally that night, before he flew to New Jersey for a fund-raiser on Thursday and more than 36 hours before the president revealed his diagnosis in a late-night tweet.

The White House also said that another member of Trump's medical team had meant to say that it was "Day 2" - not 48 hours - since the experimental drug Regeneron was administered to Trump.

Image source, Reuters

Earlier, UK PM Boris Johnson was asked by reporters if he had any advice for President Trump, having been treated for the virus in hospital himself in April. He said he was sure Trump would do "exactly what the doctors tell him to do".

Now Foreign Secretary Dominic Raab has told the online Conservative Party conference just how concerned some of the prime minister's key allies were, as his condition worsened and he was admitted to intensive care.

“It nearly took the life of our prime minister, our friend as well as our leader," said Raab, who deputised for the PM during his illness.

The foreign secretary said he was "really worried we might lose him", adding that he was also concerned for Johnson's fiancée, then pregnant with his son, Wilfred.

But he added: "I always had faith that with the outstanding NHS care he received and his fighting spirit, he’d pull through.”

Johnson was admitted to hospital on 6 April. At the time, Downing St sought to play down the move as a "precautionary step". However, he was moved to intensive care the next day, formally transferring his powers to Raab.

Two days later Johnson left intensive care and was said to be in "extremely good spirits", but it was not until 26 April that he returned to work.

It was only much later that he admitted how badly the virus had affected him.

Barbara Plett Usher
BBC News, Washington

Just a few months ago President Trump suggested that the election should be delayed, because of his false claim that postal voting would lead to massive fraud.

Now that he’s been infected by the coronavirus, that’s become a serious question: can a presidential election be postponed? The answer is that it can. But it would need both houses of Congress to change the law. And since they are controlled by different parties, such a scenario seems most unlikely.

But what if the president becomes incapacitated: could his name be removed from the ballot? That would also be very difficult, maybe impossible, because the ballots are already out and people have started voting.

However, in the United States the people do not directly choose the president. They vote for electors, who vote for the president – each state has the same number of electors as it has representatives in Congress. Theoretically, they could end up selecting the Republican ticket, even if Mr Trump’s name had to be withdrawn.

The more immediate question of course, is what happens to the campaign. The president has clearly been grounded. For now the plan is to send others to any in-person events, such as the vice president and members of Mr Trump’s family.

The president has tweeted about his condition.

"Doctors, Nurses and ALL at the GREAT Walter Reed Medical Center, and others from likewise incredible institutions who have joined them, are AMAZING!!!" the tweet read.

"Tremendous progress has been made over the last 6 months in fighting this PLAGUE. With their help, I am feeling well!"

Image source, Reuters

The US Senate will postpone its work in full session until 19 October, Senate Republican Majority Leader Mitch McConnell has said. Three senators have now tested positive for the coronavirus in the past few days.

However, Sen McConnell said the work of the Senate Judiciary Committee would continue. The committee will be examining the nomination to the Supreme Court of Judge Amy Coney Barrett, a favourite with conservatives, who has raised alarm among progressives for her opposition to abortion rights.

Two of the infected senators, Mike Lee of Utah and Thom Tillis of North Carolina, both Republicans, are members of the committee.

The full Senate had been discussing a relief package of coronavirus aid to help Americans who've lost their jobs or are suffering economically during the pandemic. The postponement also means there'll be no agreement on measures until the second half of October at the earliest.

Several US media are now reporting that President Trump was given oxygen at the White House before he left for hospital.

The New York Times quotes two people as saying that Trump had trouble breathing on Friday and "his oxygen level dropped", leading doctors to give him supplemental oxygen.

AP news agency is also reporting that he was given oxygen at the White House, quoting a source.

At a briefing on Trump's condition earlier, his physician Dr Sean Conley said he wasn't on oxygen at the moment. But his doctors refused repeatedly to rule out that he had ever been on supplementary oxygen.

Anthony Zurcher
BBC North America reporter

One would imagine the purpose of the White House medical team’s Saturday morning press conference was to reassure the public that the president is doing well and that the nation’s top medical experts are on top of the situation.

Instead, they created more confusion.

Sean Conley, the president’s physician, said Trump was diagnosed “72 hours ago” – which would be Wednesday morning. That’s before the president travelled to Minnesota for a campaign rally that night, before he flew to New Jersey for a fund-raiser on Thursday and more than 36 hours before the president revealed his coronavirus diagnosis to the world in a late-night tweet.

The timeline is further muddied by the revelation that the president was given an antiviral treatment sometime on Thursday – also before his announcement.

Conley tried to paint a positive picture of the president’s current medical condition, although he was evasive about whether Trump had ever been given oxygen to assist his breathing.

And then, just minutes after the press conference concluded, an official struck a very different tone, telling the gathered press “the president’s vitals over the last 24 hours were very concerning and the next 48 hours will be critical in terms of his care”.

The White House and the president’s medical team have some explaining to do. They have since said the physicians misspoke, but that is unlikely to put concerns to rest that the president may have knowingly put more people at risk by travelling when he knew he might have Covid-19.

In situations like this, trust is an invaluable commodity – and with every misstep or misdirection, it can be squandered.

Image source, Getty Images

Throughout the campaign, we've heard from our BBC voter panel: a mix of Americans giving their views on who they'll vote for and why.

Would you like to be one of them? What are the most important issues to you? Why?

If you'd like to get involved, find out more here.

And if you'd like to see what some of the older members of our voter panel said about Trump's illness, click here.

Image source, Reuters

Several US media outlets are now quoting an unnamed source with knowledge of the president's health as saying that his vital signs were "very concerning" over the past 24 hours.

The source says the president is not yet on a path to recovery and the next 48 hours will be critical for him.

The assessment appears to contradict Trump's physician, who earlier said the president was doing "very well" and was not on oxygen or having difficulty breathing.

The source told a White House pool report: "The President's vitals over last 24 hours were very concerning and the next 48 hours will be critical in terms of his care. We are still not on a clear path to a full recovery."

In an intriguing bit of detail, White House reporters say an official gave them a note with the comment on it.

CNBC reporter Eamonn Javers says the source spoke to one or more reporters who decided the source was credible and passed the quote on to the wider White House press corps.

Image source, Reuters

There's some confusion about how long President Trump has known he had the virus, following the briefing by his doctors.

The president first tweeted late on Thursday night, saying he had tested positive. On Friday afternoon, he was flown to hospital.

However, his doctors said on Saturday that they were "72 hours into the diagnosis". That would mean a diagnosis on Wednesday, nearly 36 hours earlier than announced.

In that time, Mr Trump held a press briefing without wearing a mask. He flew to Minnesota on Wednesday for a rally with thousands of supporters on Wedneday night.

On Thursday, he was at his golf club in New Jersey for a fundraiser - again, with very few masks in evidence.

Eyes will be turning to the White House for clarification on the exact timeline here.

Republican Senator for Florida Rick Scott has clarified that he does not have Covid-19 after earlier telling Fox News that he had tested positive for the virus.

Scott says he "misspoke" in his interview with the channel.

In the past two days, three Republican senators have confirmed that they have Covid-19, along with the president, his wife and several White House figures.

Image source, EPA

Former Republican governor of New Jersey Chris Christie - who was among several people to help Donald Trump prepare for Tuesday's debate against Joe Biden - says he has tested positive for Covid-19 and will receive medical attention.

Christie - who left the governorship in 2018 - said on Friday that he had no symptoms and felt "fine". An earlier test on Tuesday was negative, he said.

On Friday he told US media that no one had been wearing masks when he and others sat down with Trump to prepare for Tuesday's televised debate.

"No one was wearing masks in the room when we were prepping the president during that period of time. And the group was about five or six people, in total," he said.

Those present reportedly include Trump, his aide Hope Hicks and his campaign manager Bill Stepien - who have all tested positive - as well as his lawyer Rudy Giuliani and adviser Stephen Miller.

The doctors at the Walter Reed hospital are very upbeat about President Trump's condition. However, they don't know how long he'll have to stay there.

Trump is being closely monitored for any complications. He's fever-free, and his cough and nasal congestion are improving.

"More than anything, he's felt run down," Dr Sean Conley told journalists.

Starting the briefing, Mr Trump's personal doctor said they had decided to move the president to the hospital out of caution.

"Just 72 hours into the diagnosis now, the first week of Covid - in particular days seven to 10 - are the most critical in determining the likely course of this illness," Dr Sean Conley said.

But as BBC North American editor Jon Sopel pointed out on Twitter, Mr Trump only confirmed his positive test early on Friday morning - roughly 36 hours ago.

In their first update since he entered Walter Reed hospital on Friday evening, President Trump's doctors appeared in masks and white coats to share his status - saying he is being closely monitored, but they are extremely happy with his progress.

Despite his diagnosis, President Trump is continuing to work from hospital, his doctors say. The presidential suite at the Walter Reed hospital is equipped with an office.

"And the president this morning is not on oxygen, not having difficulty breathing or walking around the White House medical unit upstairs," Dr Sean Conley, Trump's personal doctor, told journalists.

The president is taking Remdesivir, which has been shown to shorten the recovery time from the coronavirus. He'll complete a five-day course of treatment, his doctors say.

President Trump asked about the drug hydroxychloroquine, but is not taking it at this time, his doctors say. Early in the pandemic, he touted the medicine as a treatment for coronavirus - a recommendation not borne out by medical research.

First Lady Melania Trump, who has also tested positive, remains at the White House and is not being treated at the Walter Reed military hospital.

She is "doing great", the doctors say, and is "convalescing at home".

President Trump has been receiving "outstanding multidisciplinary care", his doctors say.

They are "monitoring him very closely for any evidence of complications" but say he hasn't had any fever since Friday morning.

Trump isn't on oxygen at the moment. However, his doctors refused repeatedly to rule out that he had ever been on supplementary oxygen.

He's receiving both the drug Remdesivir and an experimental treatment. They tackle different aspects of the disease, the doctors say.

They say they are "maximising all aspects of his care" and "don't want to hold anything back".