President Donald Trump and first lady Melania Trump tested positive for coronavirus, the President announced early Friday morning, an extraordinary development coming months into a global pandemic and in the final stretch of his reelection campaign.

In a memo issued to reporters around 1 a.m. ET, the President’s physician, Navy Commander Dr. Sean Conley, wrote 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 diagnosis amounts to the most serious known health threat to a sitting American president in decades. At 74 years old, 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, as reported by CNN.

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 am Friday.

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 did not speak to reporters as he walked into his residence.