Democratic challenger Joe Biden, who spent 90 minutes on stage with Trump in their Tuesday debate, tested negative.

No one knows exactly what comes next.

At the least, the development focuses the campaign right where Biden has put his emphasis for months: on Trump's response to a pandemic that has killed more than 200,000 people in the US.

And for the short term, it's grounded Trump in a quarantine, denying him the large public rallies that fuel his campaign.

The stunning development injected even deeper uncertainty into an election already plagued by the pandemic, deep economic anxiety and sweeping civil unrest.

Just a month before election day, the country entered truly uncharted territory that has the potential to rattle global markets and political debates around the world.

"It's a reminder that the American presidency is bigger than any one person, given the reach and depth this news has," said Karen Finney, a Democratic consultant and top adviser to Hillary Clinton's 2016 campaign.

She said the immediate focus should be on the Trumps' health.

But she added the development proves that not even Trump, no matter his talents for dictating headlines and framing events, can control a pandemic.

Trump tweeted on Friday that he'd begin quarantining and recovery.

White House chief of staff Mark Meadows said on Friday that the president is resting, with mild symptoms.

The president has cancelled his weekend itinerary in Wisconsin, which is one of the three Great Lakes states Trump won by less than 1 percentage point in 2016 on his way to the presidency.

With election day just 32 days away, many people in the US have already begun voting in several states and tens of millions will receive absentee mail-in ballots or be eligible for in-person early voting in the coming weeks.

Strategists in both parties acknowledged the timing is bad for the Republican president.

"Trump's main advantages, including incumbency, have been removed. Rallies, his main vehicle for mobilising his base, will no longer be possible. Fly-bys with Air Force One as a backdrop are gone," said Republican strategist Rick Tyler, a frequent Trump critic.

He said that Trump's infection also "fundamentally undercuts his entire campaign strategy, which was to ignore the pandemic and make unsubstantiated claims that we've turned the corner and making an economic comeback."

The news also raises new questions about Biden's plans.

He was scheduled to travel to Michigan on Friday but the campaign appeared to be reassessing its plans as it awaits his test results.

Biden's running mate, Kamala Harris, was slated for a trip to Las Vegas.

The campaign said Harris has tested negative for the virus.

The Democratic nominee has been much more cautious in his travel itinerary than Trump, with fewer public events and all of them following social distancing guidelines.

Only Thursday did Biden's campaign announce that it would resume door-to-door canvassing in addition to its phone and digital outreach to voters.

Biden's primary care doctor, Kevin O'Connor, announced the results of his latest COVID-19 test Friday afternoon.

"I'm happy to report that Jill and I have tested negative for COVID," Biden tweeted.

"Thank you to everyone for your messages of concern. I hope this serves as a reminder: wear a mask, keep social distance, and wash your hands."

Trump and Biden did not shake hands during their debate but stood without masks about 3 metres apart for the 90-minute event.

Two additional debates are scheduled for October 15 and October 22.

The Commission on Presidential Debates has not yet commented on any changes in the debate schedule or health protocols.

Both men are in high-risk categories for COVID-19 complications.

Trump is 74 years old and Biden is 77 years old.

Australian Associated Press