It marks an unprecedented turn of events for an election campaign nearing its climax - and a president who spent months downplaying the pandemic’s threat to the American public.
Although his physician, Sean Connelly, has said the president was in good health with no symptoms reported, the 74-year-old will now spend two weeks in lockdown at the White House.
Should Mr Trump develop complications, however, it could potentially force him to withdraw from the race and lead to the Republican party naming someone else on the ballot.
And whilst the 25th Amendment of the constitution sets out how a vice-president assumes power when a president is incapacitated - what happens when a presidential candidate becomes ill or withdraws is lesser known.
Should Mr Trump be unable to continue as the Republican nominee, New York University law professor Richard Piles told the Washington Post in May the Republican national committee would assume control.
“This puts the ball in the hands of the ‘national political parties’, which for this purpose means the legal entities known as the Democratic and Republican national committees,” said the professor.
In this situation, an emergency Republican convention would be held where members from each state or territory vote on the new nominee - casting the same number of votes as they are entitled to at an ordinary convention.
In addition to the new nominee, explained Mr Piles, “the parties would now have to replace the name of their dead candidate on each state’s ballot with that of the new candidate.”
Therein lies the problem, with Mr Trump’s coronavirus diagnosis coming less than a month before the election takes place - and with millions of ballots cast in what has already become a contested race to the White House.
As Rick Hasen, a law professor at the University of California, wrote on that outcome, “what’s most likely [is] that the election would take place on time with the deceased or incapacitated candidate’s name on the ballot,”
“Then there would be a question if [state] legislatures would allow presidential electors of each state to vote for someone other than the deceased candidate,” explained Mr Hasen.
Typically, the Electoral College system would see each state count its popular votes according to state laws, and then designate its presidential electors.
In most states, the candidate with the most votes will receive all of the state’s electors, who pledge to vote for the winner when they meet in December to cast their votes in the Electoral College.
That, argued Mr Piles to the Washington Post, rested on whether the Supreme Court decided “whether it is constitutional for states to ‘bind’ their electors to vote for the candidate who won the popular vote in that state” in the event that the candidate has withdrawn - or died.
And in that case, most state laws “do not specify whether an elector must still vote for a now-dead candidate and, if not, who they must or can vote for instead,” added the New York university professor.
“When these laws were written, state legislatures were not thinking about this remote possibility. This is a glitch: States that bind electors should amend these laws to specify what an elector can or must do in this circumstance,“ he added.
Still, the Supreme Court has - as recently as this summer - ruled against three so-called “faithless electors” to the Electoral College who argued they could vote for any candidate following the 2016 race.
“Three Washington electors, Peter Chiafalo, Levi Guerra, and Esther John (the Electors), violated their pledges to support Hillary Clinton in the 2016 presidential election,” having refused to support the candidate - against precedent.
So, in any situation where either the Republican nominee, Mr Trump, or his Democratic opponent, Mr Biden, withdraw or become ill between now and December, there is at least some legal precedent as to what happens with the election outcome.