President Trump at a rally in Duluth, Minn., on Wednesday.
Credit…Erin Schaff/The New York Times

President Trump’s announcement that he had tested positive for the coronavirus caused global financial markets to plunge and set off a wave of reactions from leaders around the world.

Futures markets were predicting that Wall Street would open 1.7 percent lower. European futures fell too, as did yields on U.S. Treasury bonds.

Prime Minister Narendra Modi of India, was among the first global leaders to weigh in on the news.

“Wishing my friend @POTUS @realDonaldTrump and @FLOTUS a quick recovery and good health,” Mr. Modi wrote on Twitter, using acronyms for the president and first lady of the United States.

Other world leaders, including President Jair Bolsonaro of Brazil and Prime Minister Boris Johnson have tested positive for the virus, as did Sophie Grégoire Trudeau, the wife of Canada’s prime minister.

Credit…Tom Brenner for The New York Times

WASHINGTON — President Trump said early Friday that he and the first lady have tested positive for the coronavirus, throwing the nation’s leadership into uncertainty and escalating the crisis posed by a pandemic that has already killed more than 207,000 Americans and devastated the economy.

“Tonight, @FLOTUS and I tested positive for COVID-19,” Mr. Trump wrote on Twitter. “We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!”

Mr. Trump received the test results after one of his closest advisers, Hope Hicks, became infected, bringing the virus into his inner circle and underscoring the difficulty of containing it even with the resources of a president. Mr. Trump has for months played down the severity of the virus and told a political dinner just Thursday night that “the end of the pandemic is in sight.”

Mr. Trump’s positive test result could pose immediate difficulties for the future of his campaign against former Vice President Joseph R. Biden Jr., his Democratic challenger, with just 33 days before the election on Nov. 3. Even if Mr. Trump, 74, remains asymptomatic, he will have to withdraw from the campaign trail and stay isolated in the White House for an unknown period of time. If he becomes sick, it could raise questions about whether he should remain on the ballot at all.

Even if he does not become seriously ill, the positive test could prove devastating to his political fortunes given his months of diminishing the seriousness of the pandemic even as the virus was still ravaging the country and killing about 1,000 more Americans every day. He has repeatedly predicted the virus “is going to disappear,” asserted that it was under control and insisted that the country was “rounding the corner” to the end of the crisis. He has scorned scientists, saying they were mistaken on the severity of the situation.

Mr. Trump has refused for months to wear a mask in public on all but a few occasions and repeatedly questioned their effectiveness while mocking Mr. Biden for wearing one. Trailing in the polls, the president in recent weeks increasingly held crowded campaign events in defiance of public health guidelines and sometimes state and local governments.

Secret Service agents at President Trump’s campaign rally at Wittman Airport in Oshkosh, Wis., last month.
Credit…Doug Mills/The New York Times

The Secret Service sustained a coronavirus outbreak in August at its training facility in Maryland, evidence of just how close the pandemic has been to President Trump in the weeks before he was infected.

At least 11 employees at the center in rural Maryland tested positive for the virus even after it was closed for several months to mitigate transmissions, according to officials briefed on the matter.

Some of the personnel are believed to have contracted the virus during training exercises or at a graduation celebration at a nearby hotel where participants did not practice social distancing, the people said.

The agency declined to discuss the specifics of the outbreak, but it said in a statement that it “has taken significant precautions at its training center to protect the health and welfare of its trainees and training staff.”

The details about the problem at the center were uncovered by the Project on Government Oversight, an independent watchdog group in Washington. The organization brought the information about the outbreak to The New York Times, which independently confirmed details of it with people briefed on the matter. Those people spoke on the condition of anonymity because they did not want to be identified discussing delicate personnel matters.

The news early Friday morning that the president and the first lady had tested positive for the coronavirus gave the Secret Service outbreak new relevance. Hours before that announcement, the White House confirmed that Hope Hicks, a close adviser to Mr. Trump, had been infected.