Video report by ITV News Washington Correspondent Robert Moore
US President Donald Trump and First Lady Melania Trump are currently experiencing mild symptoms after testing positive for coronavirus.
Democratic presidential nominee Joe Biden, who has tested negative for Covid-19, said it served as a "bracing reminder to all of us that we have to take this virus seriously", adding he and wife Jill are praying the Trumps make a quick and full recovery.
He encouraged everyone to follow the science, practice social distancing and wear a mask, adding "it's not about being a tough guy."
"We can save 100,000 American lives in the next 100 days alone if everyone wore a mask in public," Mr Biden said, citing researchers from the University of Washington.
The president and his wife are being monitored by doctors, but they were said to be in "good spirits" with White House staff insisting Mr Trump is still "hard at work".
All campaign events featuring president and his family have been postponed after he and the First Lady tested positive.
Mr Biden confirmed earlier both he and wife Jill have tested negative for Covid-19 - they were tested in light of the president's confirmed coronavirus diagnosis.
The former vice president appeared alongside Mr Trump at Tuesday’s first presidential debate.
Though the candidates did not shake hands and stood at a safe social distance, they were onstage together for more than an hour and a half, without masks.
Vice-president Mike Pence - who has tested negative for Covid - will continue on the campaign trail in Mr Trump's absence.
White House Press Secretary Kayleigh McEnany told reporters on Friday: "I'm not going to get into the president's symptoms exactly, he's had mild symptoms."
She added: "He's been hard at work, we've had to slow him down a little bit."
Ms McEnany said the White House issued news of the president's positive test "within an hour" of him having received the news.
White House chief of staff Mark Meadows echoed Ms McEnany's message.
He told reporters the president remains "energetic and was continuing to work despite his illness."
He added: “The president does have mild symptoms. The doctors continue to monitor both his health and the health of the first lady.”
The president's youngest son, Barron, has received a negative test result "and all precautions are being taken to ensure he’s kept safe and healthy,” said Stephanie Grisham, the first lady’s spokeswoman.
Mr Trump revealed him and his wife had tested positive for Covid-19 in a tweet early on Friday morning.
The first lady thanked people for their support in a tweet on Friday afternoon."Thank you for the love you are sending our way. I have mild symptoms but overall feeling good. I am looking forward to a speedy recovery," she wrote.
The Trumps' positive tests follow the announcement on Thursday that the president's senior aide, Hope Hicks, had tested positive for Covid.
Mr Trump initially tweeted to say he and the First Lady were awaiting test results in quarantine, but two hours later confirmed they had both tested positive for the virus.
He tweeted: “Tonight, @FLOTUS and I tested positive for COVID-19. We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!”
The stunning development comes just a month before the US election on November 3.
Mr Trump is now expected to be in quarantine for some two weeks while facing treatment for the virus.
House Speaker Nancy Pelosi said she was praying for the president and hopes his testing positive might be a “learning experience” about the virus.
“Let us all pray for the president’s health,” Pelosi said on MSNBC. She added, “This is tragic, It is very sad.”
The speaker said she was tested out of caution and is awaiting results. But said warned against “brazen” behavior that allowed “something like this to happen".
The World Health Organization's director-general wished the couple "a full and swift" recovery.
Speaking from Geneva, Tedros Adhanom Ghebreyesus said his prayers are with the president and the first lady.
While Prime Minister Boris Johnson, who was admitted to intensive care in April after testing positive for Covid-19, wished the president and his wife well.
Mr Johnson said: "Well, obviously, I think we all want to send our best wishes to the president and the first lady, and I have done that this morning as you can imagine, and I’m sure that they will both stage a very strong recovery.”
Mr Trump is 74-years-old, putting him at higher risk of serious complications from the virus.
The positive tests for the Trumps come as stark reminders of the continued coronavirus pandemic, even as Mr Trump has tried desperately to suggest it no longer poses a danger.
The USA currently has the highest number of confirmed cases worldwide, and the highest death toll of any country across the globe.
Since the virus emerged at the start of the year Mr Trump, the White House and his campaign have played down the threat and refused to abide by basic public health guidelines.
The president refused to appear in public wearing a face mask at the height of pandemic before making a U-turn on the matter - though none of his team were wearing face coverings at the first presidential debate on Tuesday.
Mr Trump has continued to hold campaign rallies that draw thousands of supporters with a lack of social distancing visible in the audiences at his events.
The president continued to shake hands with visitors long after public health officials were warning against it. He also initially resisted being tested.
The Pennsylvania Department of Health is encouraging people who were at Mr Trump's rally at Harrisburg International Airport on Saturday to get tested if they are feeling unwell, in light of the president's diagnosis.
Larry Kudlow, White House economic adviser, told reporters on Friday there had been no change in White House coronavirus protocol since the president's positive test.
"Our protocols have been in place from day one and remain in place," he said.
ITV News Washington Correspondent Robert Moore reacts to the development
The political implications of this will depend on several factors.
How long will the President stay off the campaign trail? Would a negative result be enough for him to resume his schedule? The next presidential debate is two weeks away, on October 15th, in Miami. Donald Trump is 74-years-old and significantly overweight, if not obese, two widely acknowledged risk factors. It seems unlikely that Donald Trump - if he was infected by Tuesday - could have passed it on to Joe Biden.
According to the strict protocols of the event, the candidates were kept apart and never even shook hands. In fact, even as a journalist accredited to the debate, I was subjected to a Covid test, administered nearby, before I could enter the debate zone. This strange presidential election campaign has taken another bizarre twist. The President who said Covid would quickly go away, and who has been largely dismissive of masks, and who even speculated about injecting bleach as a cure, has spent an anxious night in the White House.
Who is Hope Hicks?
Ms Hicks who serves as counsellor to the president and travelled with him to a rally on Wednesday, tested positive on Thursday.
The 31-year-old is the closest aide to Mr Trump to test positive so far.
Ms Hicks travelled with the president multiple times this week, including aboard Marine One, the presidential helicopter, for a Minnesota rally on Wednesday, and aboard Air Force One to Tuesday night’s first presidential debate against Joe Biden.
Multiple White House staffers have tested positive for the virus, including Katie Miller, Vice President Mike Pence’s press secretary, as well as one of the president’s personal valets.
Utah Sen. Mike Lee says he too has tested positive for the coronavirus after experiencing “symptoms consistent with longtime allergies.”
Lee visited the White House on Saturday for President Donald Trump’s announcement that he had nominated Judge Amy Comey Barrett to the Supreme Court.
The news was certain to rattle an already shaken nation still grappling with how to safely reopen while avoiding further spikes of the disease.
The White House has access to near-unlimited resources, including a constant supply of quick-result tests, and still failed to keep the president safe.
Mr Trump, the vice president and other senior staff have been tested for Covid-19 daily since two people who work at the White House complex tested positive in early May, prompting the White House to step up precautions.
Everyone who comes into contact with the president also receives a quick-result test.
Yet since the early days of the pandemic, experts have questioned the health and safety protocols at the White House and asked why more was not being done to protect the commander in chief.
Hope Hicks and president Trump board a helicopter together with other White House staff:
In a statement, White House spokesman Judd Deere said: “The president takes the health and safety of himself and everyone who works in support of him and the American people very seriously.”
“White House Operations collaborates with the physician to the president and the White House Military Office to ensure all plans and procedures incorporate current CDC guidance and best practices for limiting Covid-19 exposure to the greatest extent possible both on complex and when the president is travelling,” he added.
Mr Trump is not the only major world leader known to have contracted the virus.
Boris Johnson was given oxygen and watched around the clock by medical workers during his three nights in ICU.German Chancellor Angela Merkel self-isolated after a doctor who gave her a vaccination tested positive for the virus, and Canadian Prime Minister Justin Trudeau worked from home after his wife fell ill.