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. 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.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.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 had said late Thursday night that he planned to quarantine after one of his closest aides tested positive for the infection.“Hope Hicks, who has been working so hard without even taking a small break, has just tested positive for Covid 19. Terrible! The First Lady and I are waiting for our test results. In the meantime, we will begin our quarantine process!” Trump tweeted Thursday night.While the White House frequently touts its extensive testing regime for the President and those who come into close proximity with him — aides have deemed him the “most tested man in the country” — he has eschewed practices like social distancing and mask wearing.At Tuesday evening’s presidential debate, the President mocked his rival for frequently wearing a mask.