Dow futures were last down 439 points, or 1.6 per cent. S&P 500 futures were down 2 per cent and Nasdaq futures were down 1.8 per cent.

Twitter : Donald J. Trump

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

Oil prices have also fallen upon news of the President testing positive.

Crude oil dropped almost 3 per cent in the 10 minutes following Mr Trump's tweet.

But those losses quickly accelerated after Trump said those tests were positive.

Trump was last seen by reporters returning to the White House today and looked to be in good health.

Trump is 74 years old, putting him at higher risk of serious complications from a virus that has now killed more than 200,000 people nationwide.

Mr Trump announced today he and Ms Trump were beginning a "quarantine process" after Ms Hicks came down with the virus, though it wasn't clear what that entailed.

Twitter : Melania Trump

As too many Americans have done this year, @potus & I are quarantining at home after testing positive for COVID-19. We are feeling good & I have postponed all upcoming engagements. Please be sure you are staying safe & we will all get through this together.

It can take days for an infection to be detectable by a test.

The diagnosis marks a major blow for a president who has been trying desperately to convince the American public that the worst of the pandemic is behind them even as cases continue to rise with less than four months before Election Day.

And it stands as the most serious known public health scare encountered by any sitting American president in recent history.

Symptoms of COVID-19 can include fever, cough and breathing trouble. Most people develop only mild symptoms.

But some people, usually those with other medical complications, develop more severe symptoms, including pneumonia, which can be fatal.

In an interview with Fox News host Sean Hannity today, Mr Trump said he was awaiting results of a COVID-19 test.