The United States 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

Tonight, @FLOTUS and I tested positive for COVID-19. We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!

— Donald J. Trump (@realDonaldTrump) October 2, 2020

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.