The British political system is different. As a Conservative frontbencher, Michael Gove, said at the time Mr Johnson relinquished power to Mr Raab: “The Cabinet is the supreme decision-making body.”
The United States does not have a Westminster system of government. Congress is entirely separate from the executive branch, headed by Mr Trump.
So, should America face the sort of crisis Britain dealt with all those months ago, the procedure for filling the power vacuum is not the same.
The relevant part of the US Constitution is the 25th Amendment.
“In case of the removal of the president from office, or of his death, resignation or inability to discharge the powers and duties of the said office, the same shall devolve on the vice president,” it states.
That’s pretty straightforward. Should Mr Trump become incapable of doing the job, Vice President Mike Pence will take over.
The most likely way this would occur is through Mr Trump himself declaring himself “unable to discharge the powers and duties” of his office, in a letter to both the president pro tempore of the US Senate and the Speaker of the House of Representatives.
Mr Pence would become acting president, and Mr Trump would be able to take back power at any time by writing letters to the same two public officials, declaring himself able to resume his duties.
This scenario has unfolded three times in US history.
In 1985, Ronald Reagan had surgery to remove a small growth in his colon. His vice president, George H.W. Bush, took over for a short time.
In 2002, George W. Bush needed a colonoscopy, and briefly handed power to vice president Dick Cheney while he was under anaesthesia. It took no longer than two hours.
In 2007, Mr Bush had another colonoscopy. Again, Mr Cheney acted as president for a couple of hours.
A separate provision, which is unlikely to ever be used, allows a majority of “the principal officers of the executive departments”, or in other words the Cabinet, to declare the President unfit to do his job and eject him from office.
“Karen and I send our love and prayers to our dear friends President Donald Trump and First Lady Melania Trump,” Mr Pence said in a statement after Mr Trump’s positive test.
“We join millions across America praying for their full and swift recovery. God bless you President Trump and our wonderful First Lady Melania.”
If you want to get even more outlandish, what happens in a hypothetical scenario in which Mr Trump and Mr Pence are both incapacitated at the same time?
In that case, House Speaker Nancy Pelosi, a Democrat, is next in the line of succession. So she would become acting president.
Mr Trump seems confident that none of these hypotheticals will come true.
“We will get through this together,” he said today after receiving his diagnosis.
The President said he and Mrs Trump would begin their quarantine and recovery process “immediately”.
Mrs Trump also posted a statement, saying she and her husband were “feeling good”.
“As too many Americans have done this year, POTUS and I are quarantining at home after testing positive for COVID-19,” she said.
“We are feeling good and I have postponed all upcoming engagements. Please be sure you are staying safe and we will all get through this together.”