US President Donald Trump has been flown to hospital for treatment after testing positive for coronavirus.
Mr Trump tweeted "going well, I think!" after his arrival. US media say his symptoms include a low-grade fever.
He has so far been treated with an experimental drug cocktail injection and the antiviral medication remdesivir after both he and First Lady Melania Trump tested positive for Covid-19.
In exactly one month, Mr Trump faces Joe Biden in the presidential election.
The president's diagnosis has upended his campaign and also cast doubt on his attempt to get a new Supreme Court judge confirmed before polling day.
The latest update from Mr Trump's physician, Sean Conley, in a memorandum late on Friday, read: "I am happy to report the president is doing very well." He said the president was not in need of supplemental oxygen.
Mr Trump was taken to hospital "out of an abundance of caution" with "mild symptoms" and would be there for the "next few days", the White House said.
The list of other people to have tested positive around Mr Trump include close aide Hope Hicks - believed to be the first to show symptoms - campaign manager Bill Stepien and former White House counsellor Kellyanne Conway. Republican Senators Mike Lee and Thom Tillis have also tested positive.
Mr Trump remains in charge. Vice-President Mike Pence, to whom under the constitution the president would transfer power temporarily should he become too ill to carry out his duties, tested negative.
How did the president look on Friday?
Wearing a mask and suit, Mr Trump walked out across the White House lawn on Friday at 18:15 (22:15 GMT) to his helicopter, Marine One, for the short flight to Walter Reed National Military Medical Center close to Washington DC.
He waved and gave a thumbs-up to reporters but said nothing before boarding the aircraft.
In an 18-second video posted to Twitter, Mr Trump said: "I think I'm doing very well. But we're going to make sure that things work out. The first lady is doing very well. So thank you very much."
The president's children, Ivanka and Eric, retweeted his post, praising him as a "warrior". Ms Trump added: "I love you dad."
Donald Jr said his father was "obviously taking it very seriously".
Mr Trump was admitted to the presidential suite at Walter Reed, which is where US presidents usually have their annual check-up.
Shortly before midnight, he tweeted again: "Going well, I think! Thank you to all. LOVE!!!"
UK PM Boris Johnson, who spent time in intensive care with the virus in April, said he had no doubt Mr Trump would make a "very strong recovery".
What treatment has the president received?
His doctor, Sean Conley, said the president was "not requiring any supplemental oxygen, but in consultation with specialists we have elected to initiate remdesivir therapy. He has completed his first dose and is resting comfortably".
Tests have shown remdesivir, originally developed as an Ebola treatment, disrupts the virus's ability to duplicate and can cut the duration of symptoms.
Earlier on Friday, Dr Conley said the president had "as a precautionary measure received an 8g dose of Regeneron's antibody cocktail" at the White House.
The antibodies stick to the coronavirus, protecting the body's cells and making the virus more visible to the immune system.
However, the treatment is still experimental and has not yet been approved by regulators. Although there is a lot of hope behind it, some doctors have expressed surprise at its use on the president at this stage.
Mr Trump was also taking zinc, vitamin D, famotidine, melatonin and aspirin, Dr Conley said.
The president, being 74, a man and someone categorised as obese, is in a higher-risk category for Covid-19.
The first lady, who is 50, was said to be "well with only a mild cough and headache".
On Friday afternoon, Mrs Trump said she was experiencing mild symptoms but was "feeling good" and "looking forward to a speedy recovery".
The remainder of the Trump family, including the first couple's son Barron, who also lives at the White House, tested negative.
Covid cluster at the heart of government
Analysis by Peter Bowes, BBC North America correspondent
This is a crisis that's escalating by the hour.
With the news that another member of Trump's inner circle, Kellyanne Conway, has tested positive, along with two Republican senators, there's a growing realisation that a Covid cluster has penetrated the heart of US government.
Like Ms Conway, Senators Mike Lee of Utah and Thom Tillis of North Carolina attended the White House event where the president announced his nomination of Amy Coney Barrett to the Supreme Court.
Chuck Schumer, the Democratic leader in the Senate, said it would be "irresponsible and dangerous" to move forward with Amy Coney Barrett's confirmation hearings.
It would be another huge blow to the president if the proceedings were delayed, although Senate leader Mitch McConnell has said they will continue with expanded use of remote hearings.
How have Democrats reacted?
Mr Biden, the Democratic nominee, and his wife Jill tested negative on Friday. The Bidens wished the presidential couple a speedy recovery.
This cannot be a partisan moment.— Joe Biden (@JoeBiden) October 2, 2020
It must be an American moment.
We have to come together as a nation.
The Democratic campaign is temporarily taking down all its negative ads regarding Mr Trump.
Mr Biden, on the campaign trail on Friday in Grand Rapids, Michigan, added: "It's a bracing reminder to all of us that we have to take this virus seriously. It's not going away automatically."
Since the onset of the pandemic, Democrats have criticised the president for flouting basic health guidelines - such as social distancing and wearing a facial covering in public - including those promoted by his own administration.
Mr Trump has also continued to hold large campaign rallies, gathering thousands of supporters, often without masks.
The Republican president also poked fun at Mr Biden during their live TV debate on Tuesday, saying: "I don't wear masks like him. Every time you see him, he's got a mask."
The most powerful elected Democrat, House of Representatives Speaker Nancy Pelosi, said her prayers for Mr Trump had "intensified" and she hoped his diagnosis would be "a learning experience" for the country.
The US has had more Covid-19 cases - 7.3 million - and more deaths - 208,000 - than any other nation, according to Johns Hopkins University research.
What of others who have tested positive?
Campaign manager Bill Stepien has "mild flu-like symptoms" and Kellyanne Conway also has "mild symptoms".
Republican National Committee Chairwoman Ronna McDaniel, who was with Mr Trump a week ago, has tested positive, as has Rev John Jenkins, president of Notre Dame University, who visited the White House last weekend for the Supreme Court announcement.
There has been criticism of the president's decision to go to a fundraiser attended by dozens of people in New Jersey on Thursday, apparently when officials already knew about Hope Hicks' symptoms.
Officials said the process of tracking all Mr Trump's contacts in recent days was ongoing.
Republican Senator Ron Johnson of Wisconsin tested positive, without symptoms, on Friday. He had left two weeks of self-quarantine on Tuesday but was exposed to another infected individual shortly after returning to Washington.