President Donald Trump, who played down the threat of the coronavirus pandemic for months, says he and his wife Melania have tested positive for COVID-19 and are going into quarantine, upending the race for the White House.
"We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!" the president said in a tweet early on Friday.
Trump, 74, is at high risk because of his age and because he is considered overweight. He has remained in good health during his time in office but is not known to exercise regularly or to follow a healthy diet.
Trump understated the seriousness of the coronavirus outbreak in its early stages and has repeatedly predicted it would go away.
On Thursday night he said the end of the pandemic was in sight. He rarely wears a mask and criticises people, including Democratic presidential nominee Joe Biden, who do.
More than 200,000 people have died from COVID-19 in the US alone.
Trump, a Republican, urged US states to reopen shuttered economies even as the number of cases continued to soar and he knocked Democratic governors for putting tough measures into place to bring the virus under control.
His health scare, which rattled financial markets , shifts the 2020 presidential campaign dramatically.
With only weeks before the November 3 election, Trump will now be sidelined from doing in-person events after holding a slew of rallies with thousands of people attending, against the advice of public health professionals.
Trump's positive test also means that others at the highest levels of the US government have been exposed and may have to quarantine, too.
A White House official said early on Friday that contact tracing was under way.
Trump's physician, Sean Conley, said he expected the president to carry out his duties "without disruption" while he recovers.
"The President and First Lady are both well at this time, and they plan to remain at home within the White House during their convalescence," Conley wrote in a memo.
Leaders around the world wished Trump a speedy recovery.
On Thursday night news broke that Hope Hicks, a top adviser and trusted aide, had tested positive for the virus. Hicks traveled with the president on Air Force One on Tuesday and Wednesday.
The White House issued a new schedule for Trump's activities on Friday that did not include a planned trip to Florida. Biden plans to campaign in Michigan on Friday. Both states are swing states that could help decide the election.
The president's condition is likely to bring the pandemic back to centre stage.
Trump has been criticised for questioning the efficacy of wearing a face covering and has ridiculed Biden for putting one on regularly.
Health professionals say face coverings are key to preventing the virus from spreading.
The White House lowered its precautions as Trump sought to project a return-to-normalcy message this summer. Temperature checks of everyone entering the White House complex stopped, and while coronavirus tests continued for people who came into close proximity to Trump, including some reporters, others on campus are not tested.
Trump joins a roster of other world leaders who have contracted the virus.
British Prime Minister Boris Johnson kept working in isolation for a little over a week after testing positive in late March, but then fell gravely ill and was rushed into intensive care. He spent several weeks recovering before returning to work.
Canadian Prime Minister Justin Trudeau went into quarantine in March after his wife was diagnosed with the coronavirus. Brazil's President Jair Bolsonaro called it a "little flu" after being infected in July.