President Donald Trump announced early Friday that he and his wife both tested positive for the coronavirus, an extraordinary development coming months into a global pandemic and in the final stretch of his reelection campaign in which he has flouted experts' guidance on preventing the disease's spread.

The diagnosis amounts to the most serious known health threat to a sitting American president in decades.

At 74 years old and obese, Trump falls into the highest risk category for serious complications from the disease, which has killed more than 200,000 Americans and more than 1 million people worldwide.

His infection with the disease could prove destabilizing in an already fraught political climate, and stock market futures tumbled on news of Trump's infection.

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

Later, the first lady wrote on Twitter that she and her husband were “feeling good.”

Trump was last seen in public on Thursday afternoon, returning to the White House after a fundraising trip to New Jersey. He did not appear ill, though he did not speak to reporters as he walked into his residence.

In a memo issued to reporters around 1 a.m. ET, the President's physician, Navy Cmdr. Dr. Sean Conley, wrote that he received confirmation of the positive tests on Thursday evening.

“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.

“The White House medical team and I will maintain a vigilant watch, and I appreciate the support provided by some of our country's greatest medical professionals and institutions,” Conley wrote, without elaborating what assistance was being provided to the White House.

“Rest assured I expect the President to continue carrying out his duties without disruption while recovering, and I will keep you updated on any further developments,” he wrote.

The President had said late Thursday night that he planned to quarantine after one of his closest aides, Hope Hicks, tested positive for the infection, bringing the disease into his innermost circle.

Earlier in the night, he had downplayed the virus’ continued spread.

“The end of the pandemic is in sight,” he said during prerecorded remarks at the annual Al Smith dinner, held virtually because of the health crisis.

Ignoring the science

While the White House frequently touts its extensive testing regimen for the President and those who come into close proximity with him — aides have deemed him the “most tested man in the country” — the infections underscored the shortcomings in relying on tests alone to prevent contagion.

Trump and many of his aides have eschewed practices like social distancing and mask wearing.

At Tuesday evening's presidential debate, the President mocked his rival, Democratic nominee Joe Biden, for frequently wearing a mask. After announcing the President's diagnosis, the White House issued a new schedule to reflect several canceled events on Friday, including a fundraiser in Washington and a campaign rally in Florida.

The scrubbed plans were the most immediate effects of the President's diagnosis on the presidential campaign, which has centered largely on the pandemic and the President's response to it.

Trump has frequently sought to paint a rosy picture of the current situation, including just hours before he announced his diagnosis.

Trump has also repeatedly insisted the country is “rounding the corner” on the disease and claimed the virus would “disappear,” though even health experts within his own administration said those claims did not reflect reality.

Getting infected himself with coronavirus will do little to further his claims that the disease is waning, and undercut his push for states to reopen schools and businesses.

Trump has complained during recent rallies about Democratic governors who have maintained strict lockdowns to prevent the spread of the virus.

At-risk category

Trump has blatantly disregarded medical recommendations from his own coronavirus task force during the pandemic, proceeding with a busy schedule of packed campaign rallies. Most recently, his supporters stuffed into events in Minnesota and Pennsylvania.

Those events appear on hold for now as the President stays at the White House. Long guarded about his medical history, the President's most recent physical showed he weighed 243 pounds — considered obese for his height. Still, Conley said he was in “very good health” in a readout of the exam.

Aside from Hicks, several of the President's aides have tested positive for the coronavirus since the pandemic began ravaging the country earlier this year. In May, two White House staffers, including a member of the Navy who serves as one of Trump's personal valets, tested positive for the virus, and in July a cafeteria employee on the White House grounds tested positive as well. The vice president's press secretary has tested positive, as did the President's national security adviser Robert O'Brien. The President confirmed a fourth positive case on White House grounds last month.

Trump has previously expressed concern that aides contracting the coronavirus would undercut his message that the outbreak is waning and states should accelerate reopening, according to a person who had spoken with him.

And he asked in the spring how it was possible that one of his valets — responsible for handling his food and drink — could have come into contact with him.

The White House has since gone to great lengths to shield Trump and Vice President Mike Pence from the outbreak, even as they travel to states where cases are surging.

Close aide tests positive

Hicks has traveled with the President multiple times recently, including to the debate in Cleveland on Tuesday, and was seen boarding Marine One, along with several other of the President's closest aides — Jared Kushner, Dan Scavino and Nicholas Luna — none of whom wore masks, on Wednesday as Trump was heading to a campaign rally in Minnesota.

The US Centers for Disease Control and Prevention guidelines state that a 14-day quarantine should take place after the last known exposure to someone who has tested positive for Covid-19. This is because the incubation period for the novel coronavirus can be up to two weeks.

Trump, in his Fox News interview, speculated that Hicks could have contracted the virus from an interaction with a supporter.

“She's a very warm person. She has a hard time, when soldiers and law enforcement comes up to her, you know, she wants to treat them great, not say, ‘Stay away, I can't get near you.’ It's a very, very tough disease,” he said.

A source close to Hicks told CNN that she is experiencing symptoms and is back in Washington. A source familiar with Hicks’ symptoms describes her as being achy and feeling pretty bad. CNN has reached out to Hicks for comment.

“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 spokesman Judd Deere told CNN in a statement when asked about the level of contact between Hicks and Trump.

The White House made no mention of Hicks by name, nor did it confirm she had tested positive.

“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 traveling,” Deere added.

Some White House staffers who were in close proximity were notified of the positive test result today, one official said.