WASHINGTON (AP) -- Stricken by COVID-19, a feverish and fatigued President Donald Trump was flown to a military hospital Friday night after being injected with an experimental drug combination in treatment at the White House.

In a day of whipsaw events, the president who has spent months downplaying the threat of the virus was forced to cancel all campaign events a month before the election as he fought a virus that has killed more than 205,000 Americans and is hitting others in his orbit as well.

The White House said Trump's expected stay of "a few days" at Walter Reed National Military Medical Center was precautionary  and that he would continue to work from the hospital's presidential suite, which is equipped to allow him to keep up his official duties.

Trump walked out of the White House Friday evening wearing a mask and gave a thumbs-up to reporters but did not speak before boarding Marine One. Members of the aircrew, Secret Service agents and White House staff wore face coverings to protect themselves from the president onboard the helicopter.

In a video taped before leaving for Walter Reed, Trump said, "I think I'm doing very well, but we're going to make sure that things work out." He remained fully president, all authority intact.

Just a month before the presidential election, Trump's revelation that he was positive for the virus came by tweet about 1 a.m. Friday after he had returned from a Thursday afternoon political fundraiser. He had gone ahead to the event, saying nothing to the crowd though knowing he had been exposed to an aide with the disease that has infected millions in America and killed more than a million worldwide.

First lady Melania Trump also tested positive, the president said, and several others in the White House have, too, prompting concern that the White House or even Trump himself might have spread the virus further. He said in his video that his wife was doing very well.

Trump's diagnosis came during an already turbulent period in Washington and around the world, with the US gripped in a heated presidential election amid the human and economic toll of the virus. Trump's immediate campaign events were all canceled, and his next debate with Democrat Joe Biden, scheduled for Oct. 15, is now in question.

Trump has been trying all year -- and as recently as Wednesday -- to convince the American public that the worst of the pandemic is past, and he has consistently played down concerns about being personally vulnerable. He has mostly refused to abide by basic public health guidelines -- including those issued by his own administration -- such as wearing face coverings in public and practicing social distancing. Until he tested positive, he continued to hold campaign rallies that drew thousands of often maskless supporters.

"I felt no vulnerability whatsoever," he told reporters back in May. With the election coming up in about a month, he is urging states and cities to "reopen" and reduce or eliminate shutdown rules despite continuing virus outbreaks.

The White House tried to maintain an atmosphere of business-as-usual on Friday.

"President Trump remains in good spirts, has mild symptoms, and has been working throughout the day," said press secretary Kayleigh McEnany. "Out of an abundance of caution, and at the recommendation of his physician and medical experts, the president will be working from the presidential offices at Walter Reed for the next few days."

The president's physician said in a memo that Trump received a dose of an experimental antibody combination by Regeneron that is in clinical trials. Navy Commander Dr. Sean Conley said Trump "remains fatigued but in good spirits" and that a team of experts was evaluating both the president and first lady in regard to next steps.

The first lady, who is 50, has a "mild cough and headache," Conley reported, and the remainder of the first family, including the Trumps' son Barron, who lives at the White House, tested negative.

Trump is 74 years old and clinically obese, putting him at higher risk of serious complications from a virus that has infected more than 7 million people nationwide.

Both Democratic presidential nominee Biden and his running mate Kamala Harris have tested negative, their campaign said. Vice President Mike Pence tested negative for the virus Friday morning and "remains in good health," his spokesman said. Pence was to resume his campaign schedule after his test.

Trump's Supreme Court nominee, Judge Amy Coney Barrett, who was with him and many others on Saturday and has been on Capitol Hill meeting with lawmakers, also tested negative, the White House said. It was confirmed that she had a mild case of COVID earlier this year and has now recovered.

At least two Republican lawmakers at the Saturday Rose Garden event announcing Barrett's nomination -- Utah Sen. Mike Lee and North Carolina Sen. Thom Tillis -- announced Friday they had tested positive and were isolating.

Very early Friday, after returning from the Thursday afternoon New Jersey fundraiser, Trump stunningly tweeted, "Tonight, (at) FLOTUS and I tested positive for COVID-19. We will begin our quarantine and recovery process immediately. We will get through this TOGETHER!"

Hours earlier, the White House confirmed that a top aide who had traveled with him during the week had tested positive.

White House chief of staff Mark Meadows on Friday confirmed that the White House knew Hope Hicks, the aide, had tested positive before Trump attended the fundraiser.

"I can tell you in terms of Hope Hicks, we discovered that right as Marine One was taking off yesterday," said Meadows. Several staffers were pulled from the trip, but Trump did not cancel and there was no direct evidence that her illness was connected to his.

Many White House and senior administration officials were undergoing tests Friday, but the full scale of the outbreak around the president may not be known for some time as it can take days for an infection to be detectable  by a test. Officials with the White House Medical Unit were tracing the president's contacts.

Trump's handling of the pandemic has already been a major flashpoint in his race against Biden, who spent much of the summer off the campaign trail and at his home in Delaware citing concern about the virus. Biden has since resumed a more active campaign schedule, but with small, socially distanced crowds. He also regularly wears a mask in public, something Trump mocked him for at Tuesday night's debate.

"I don't wear masks like him," Trump said. "Every time you see him, he's got a mask. He could be speaking 200 feet away from me, and he shows up with the biggest mask I've ever seen."

In a tweet Friday morning, Biden said he and his wife "send our thoughts to President Trump and First Lady Melania Trump for a swift recovery. We will continue to pray for the health and safety of the president and his family."

World leaders offered the president and first family  their best wishes  after their diagnosis, and governments used the case as a reminder for their citizens to wear masks and practice social distancing measures.

Multiple White House staffers have previously tested positive for the virus, including Pence's press secretary, Katie Miller, national security adviser Robert O'Brien and one of the president's personal valets. An RNC official confirmed Friday that Chairwoman Ronna McDaniel learned she had tested positive Wednesday afternoon. She has been at her home in Michigan since last Saturday and did not attend the debate.

It is unclear where the Trumps or Hicks caught the virus, but in a Fox interview, Trump seemed to suggest it may have been spread by someone in the military or law enforcement in greetings.

The White House began instituting a daily testing regimen for the president's senior aides after earlier positive cases close to the president. Anyone in close proximity to the president or vice president is also tested every day, including reporters.

Trump is far from the first world leader to test positive for the virus, which previously infected Brazilian President Jair Bolsonaro and British Prime Minister Boris Johnson, who spent a week in the hospital, including three nights in intensive care. Former Italian Prime Minister Silvio Berlusconi was hospitalized last month while fighting what he called a "hellish" case of COVID-19. (AP)