WASHINGTON (AP) – President Donald Trump said early Friday that he and first lady Melania Trump have tested positive for the coronavirus, a stunning announcement that plunges the country deeper into uncertainty just a month before the presidential election.

Mr. Trump said he and Mrs. Trump were quarantining after testing positive for the virus that has killed more than 205,000 Americans. The White House physician said the president is expected to continue carrying out his duties “without disruption” while recovering.

Still, Mr. Trump’s diagnosis was sure to have a destabilizing effect in Washington and around the world, raising questions about how far the virus had spread through the highest levels of the U.S. government. Hours before Mr. Trump announced he had contracted the virus, the White House said a top aide who had traveled with him during the week had tested positive.

“Tonight, @FLOTUS and I tested positive for COVID-19. We will begin our quarantine and recovery process immediately,” Mr. Trump tweeted just before 1 a.m. “We will get through this TOGETHER!”

Vice President Mike Pence tested negative for the virus on Friday morning and “remains in good health,” his spokesman said.

Mr. Trump was last seen by reporters returning to the White House on Thursday evening and did not appear visibly ill. Trump is 74 years old, putting him at higher risk of serious complications from a virus that has infected more than 7 million people nationwide.

The president’s physician said in a memo that Mr. Trump and the first lady, who is 50, “are both well at this time” and “plan to remain at home within the White House during their convalescence.”

The diagnosis could mark a devastating blow for the president’s reelection campaign. In the best of cases, if he develops no symptoms, which can include fever, cough and breathing trouble, it will likely force him off the campaign trail just weeks before the election and puts his participation in the second presidential debate, scheduled for Oct. 15 in Miami, into doubt.

Mr. Trump’s handling of the pandemic has already been a major flashpoint in his race against Democrat Joe Biden, who spent much of the summer off the campaign trail and at his home in Delaware because of the virus. Mr. Biden has since resumed a more active campaign schedule.

In a tweet Friday morning, Mr. Biden said he and his wife “send our thoughts to President Trump and First Lady Melania Trump for a swift recovery. We will continue to pray for the health and safety of the president and his family.”

It was not immediately clear whether the former vice president had been tested since appearing at the debate with Trump or whether he was taking any additional safety protocols. Trump and Biden did not shake hands during the debate, but stood without masks about 10 feet apart for the 90-minute event.

World leaders offered the president and first family their best wishes after their diagnosis, as governments used their case as a reminder for their citizens to wear masks and practice social distancing measures.

On Friday, Mr. Trump had been scheduled to receive an intelligence briefing, attend a fundraiser and hold another campaign rally in Sanford, Florida. But just after 1 a.m., the White House released a revised schedule with only one event: a phone call on “COVID-19 support to vulnerable seniors.”

The news was sure to rattle an already shaken nation still grappling with how to safely reopen the economy without driving virus transmission. 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, raising questions about how the rest of the country will be able to protect its workers, students and the public as businesses and schools reopen. U.S. stock futures fell on the news of Mr. Trump’s diagnosis.

It is unclear where the Trumps may have caught the virus, but in a Fox interview, Mr. Trump seemed to suggest it may have been spread by someone in the military or law enforcement.

“It’s very, very hard when you are with people from the military or from law enforcement, and they come over to you, and they want to hug you, and they want to kiss you,” he said, “because we really have done a good job for them. And you get close. And things happen.”

Mr. Trump is far from the first world leader to test positive for the virus, which previously infected Brazilian President Jair Bolsonaro and British Prime Minister Boris Johnson, who spent a week in the hospital, including three nights in intensive care. Former Italian Prime Minister Silvio Berlusconi was hospitalized last month while fighting what he called a “hellish” case of COVID-19.

While there is currently no indication that Mr. Trump is seriously ill, the positive test raises questions about what would happen if he were to become incapacitated due to illness.

The Constitution’s 25th Amendment spells out the procedures under which the president can declare himself “unable to discharge the powers and duties” of the presidency. If he were to make that call, Mr. Trump would transmit a written note to the Senate president pro tempore, Republican Sen. Chuck Grassley of Iowa, and House Speaker Nancy Pelosi, D-Calif. Mr. Pence would serve as acting president until Trump transmitted “a written declaration to the contrary.”

The vice president and a majority of either the Cabinet or another body established by law can also declare the president unable to discharge the powers and duties of his office, in which case Mr. Pence would “immediately assume the powers and duties of the office as Acting President” until Mr. Trump could provide a written declaration to the contrary.