US President Donald Trump has been flown to hospital less than 24 hours after testing positive for coronavirus.
The White House said the decision to transport him to Walter Reed National Military Medical Center was taken "out of an abundance of caution".
Mr Trump - who had a fever - said early on Friday he and First Lady Melania Trump had tested positive for Covid-19.
The White House said he was feeling "fatigued but in good spirits" and was experiencing mild symptoms.
The news comes just over a month before presidential elections, where he will face Democratic challenger Joe Biden.
How did the president look on Friday?
Wearing a mask and suit, Mr Trump walked out across the White House lawn on Friday afternoon to his helicopter, Marine One, for the short trip to hospital.
He waved and gave a thumbs-up to reporters but said nothing before boarding the aircraft.
In a video posted to Twitter, Mr Trump said: "I want to thank everybody for the tremendous support. I'm going to Walter Reed hospital. 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, I appreciate it, I will never forget it - thank you."
The president's children, Ivanka and Eric, retweeted his post, praising him as a "warrior". Ms Trump added: "I love you dad."
Upon arrival to Walter Reed, the president did not go to the emergency room for treatment, but went directly to the hospital's presidential suite, according to the BBC's US partner, CBS News.
What is the White House saying?
A White House spokesperson said in a statement: "President Trump remains in good spirits, has mild symptoms, and has been working throughout the day.
"Out of an abundance of caution, and at the recommendation of his physician and medical experts, the president will be working from the presidential offices at Walter Reed for the next few days.
"President Trump appreciates the outpouring of support for both he and the first lady."
Mr Trump's symptoms include a low-grade fever, according to CBS.
Walter Reed, in the Washington DC suburbs, is one of America's largest and most renowned military medical centres. It is where US presidents usually go for their annual check-ups.
White House communications director Alyssa Farah said the president had not transferred his powers to Vice-President Mike Pence.
"The president is in charge," she said.
But he pulled out of a video conference call with vulnerable seniors scheduled for Friday, leaving Mr Pence to chair the meeting.
Under the US constitution, if Mr Trump did become too ill to carry out his duties, he could hand over his powers to the vice-president temporarily. That means Mike Pence would become acting president until Mr Trump was fit again and could resume work.
A fundamentally altered contest
The US presidential election has been turned on its head.
That sentence could have been written about any number of moments in a tumultuous year in American politics, but nothing quite like this has occurred this year, this decade, this century.
Just 32 days before the presidential election, Donald Trump has tested positive for Covid-19. Given his age, 74, he is in a high-risk category for complications from the disease. At the very least, he will have to quarantine while he is treated, meaning the US presidential contest - at least his side of it - has been fundamentally altered.
The initial implications are obvious. The president's rigorous campaign schedule - which included visits to Minnesota, Pennsylvania, Virginia, Georgia, Florida and North Carolina in just the past week - is on indefinite hold.
Trump will certainly have surrogates on the trail for him, but given that he has relied heavily on his family and senior administration and campaign officials for such tasks in the past, and many of them may have to quarantine because of their own exposure to the virus, that campaign operation will be disrupted as well.
What treatment has the president already received?
Mr Trump's physician Sean Conley said in a statement earlier on Friday that the president had "as a precautionary measure received an 8g dose of Regeneron's polyclonal antibody cocktail" at the White House.
The medication is administered to help reduce virus levels and speed recovery.
He was also taking zinc, vitamin D, famotidine, melatonin and aspirin, Dr Conley said.
"As of this afternoon he remains fatigued but in good spirits," he added. The first lady, who is 50, was "well with only a mild cough and headache".
On Friday afternoon, Mrs Trump said she was experiencing mild symptoms but is "feeling good" and "looking forward to a speedy recovery".
The remainder of the Trumps' family, including the first couple's son Barron who also lives at the White House, tested negative.
How has the Biden campaign reacted?
Mr Biden himself and his wife Jill tested negative on Friday.
"I hope this serves as a reminder," the Democratic White House candidate tweeted after the result. "Wear a mask, keep social distance, and wash your hands."
The Biden campaign said it was in the process of temporarily taking down all its negative ads regarding Mr Trump.
Mr Biden's staff said he would travel to Michigan on Friday as planned for several campaign events. He and his wife have wished the presidential couple a speedy recovery.
This cannot be a partisan moment.
It must be an American moment.
We have to come together as a nation.
— Joe Biden (@JoeBiden) October 2, 2020
Speaking at a virtual campaign event for Mr Biden, former President Barack Obama also extended well wishes to Mr and Mrs Trump.
"We're all Americans and we're all human beings and we want to make sure everybody is healthy," he said.
How did the president contract the virus?
It is not known how exactly Mr and Mrs Trump became infected. On Thursday, the first couple said they intended to self-isolate after one of Mr Trump's closest aides, Hope Hicks, tested positive. Soon afterwards, they too received positive test results.
There has been criticism of Mr Trump's decision to go to a fundraiser attended by dozens of people in New Jersey on Thursday, apparently when officials already knew about Ms Hicks' symptoms.
Officials said the process of tracking all the president's contacts in recent days was ongoing, adding that Mr Trump was considering how he might address the nation or otherwise communicate with the American people on Friday.
Since the onset of the pandemic, the president has flouted 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.