His aversion shows up in meetings at the White House, on the campaign trail and at 30,000 feet. And everyone close to Trump knows the president’s true red line.

“If you’re the perpetrator of a cough or of a sneeze or any kind of thing that makes you look sick, you get that look,” said a former Trump campaign official.

“You get the scowl. You get the response of - he’ll put a hand up in a gesture of, you should be backing away from him, you should be more considerate and you should extricate yourself from the situation.”

The president’s admitted anti-germ obsession has been a fixture throughout his career - from real-estate deal rooms to casino floors - and it’s now popping up in more public ways.

It could create another round of tactile challenges as Trump launches his 2020 campaign, where he may try to steer visitors toward his signature thumbs-up selfies and away from handshakes for the next 16 months.

The president’s hatred of involuntary bacterial emissions burst into the open last month when his acting chief of staff, Mick Mulvaney, started coughing in the Oval Office while ABC’s George Stephanopoulos was interviewing Trump.

“I don’t like that, you know. I don’t like that. If you’re going to cough, please, leave the room,” Trump said before shaking his head in seeming disgust.

White House aides say the president is simply displaying common sense: Trump keeping his hands clean is a good way to avoid getting him, or his staff, sick.

Indeed, many presidents have sought to avoid germs, using hand sanitiser and taking other precautions after shaking so many hands over the course of a day. But Trump often takes the practice to an extreme.

White House staffers know that if they’re visibly sick or sound hoarse, they must steer clear of a president who doesn’t want to be around anyone who’s under the weather.

“It was serious that you shouldn’t spend much time in front of the president [if you were sick] because he would be extremely annoyed by that,” said someone close to the White House.

“He doesn’t want to get sick. ... Everyone understood that if you were sick or sounded sick, your involvement in front of the president should be extremely limited.”

Sniffling staffers have been told they should take a break to drink some tea or otherwise get cleared up before going into the Oval Office, said this person.

Trump has a long history of germaphobia, which has sometimes hurt his business.

Jack O’Donnell, who was vice-president of the Trump Plaza casino in Atlantic City, recalled how Trump didn’t do well with some customers back in the late 1980s - because he hated shaking their hands.

“Customers wanted to be around him and they didn’t understand when he didn’t shake their hand,” he said.

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“They would be like, ‘What a rude guy. I put my hand out, and he turned it down’.”

“People would approach him as he’d walk through the casino to get to the offices and he hated that they touched him,” O’Donnell said.

“He was always that way.”

On the 2016 campaign, new campaign staffers were told not to cough or sneeze if they were in a room with Trump, said a former campaign staffer, who added that Trump and germs became a running joke among staffers.

“We were surprised that somebody who had such an issue with germs would ultimately run because you have to shake so many hands,” said the former staffer.

During the campaign, spokesperson Hope Hicks often offered Trump Purell - and he made frequent use of it, according to a former campaign official.

More recently, Trump has used his germaphobia advantageously - as a shield to rebut salacious allegations in the unverified “golden shower” claims about his behaviour at the Ritz Carlton in Moscow.

During the presidential transition, right after BuzzFeed published the Christopher Steele dossier that included those allegations about Trump’s conduct in Moscow, Trump said at a combative press conference: “Does anyone really believe that story? I’m also very much of a germaphobe, by the way, believe me.”

He hasn’t changed much as president, with a very public job that entails him meeting people throughout the day, including world leaders who he must treat respectfully in diplomatic dealings.

But even when guests he knows come to the Oval Office, Trump sometimes hesitates to initiate a handshake himself, leading people to extend their hand to get one.

“When you’ve been around this guy a lot, you know how it’s going to go. You’re in there and somebody will walk in and put their hand out and you’re just thinking to yourself, ‘Uh, that’s a mistake’,” said a former campaign official.

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In the Trump White House, dining room attendants often bring out sanitary hand wipes along with dinner for the president.

A stash of Purell is kept outside the Oval Office. During the photo line at White House Christmas parties, Trump likes to use sanitiser in between shaking people’s hands, according to a former White House staffer.

Trump’s personal aide, or “body man” carries a bottle of hand sanitiser at all times.

Before Trump eats or after shaking hands during meet and greets, Trump sticks out his hand to get a squirt of it, according to a former White House official.

Trump’s germaphobia has created challenges up in the air. Even though Air Force One is a state-of-the-art aircraft, it’s still a metal tube in the sky - a prime vector for germs due to the number of people moving through tight quarters.

White House staffers avoid going near the president - and some have avoided even going on trips - if they have a cold.

And when someone coughs or sneezes in Trump’s presence on Air Force One, he’s been known to quickly assume the worst. “Are you sick?” he has asked, according to the person close to the White House.

In July 2017, during Anthony Scaramucci’s brief White House tenure, the communications director had a sore throat and was coughing on an Air Force One flight to Ohio.

Trump noticed and told Ronny Jackson, the White House physician at the time, that Scaramucci wasn’t feeling well and ordered him to go check him out.

Plenty of aides have joked that Trump is a germaphobe, Scaramucci said.

But even more, Scaramucci added: “He’s a ballbuster. If you’re standing by him, and you’re going to look at something on his desk, and you lick your index finger to open the thing to try to catch an edge on the paper, he’ll smack your hand and be like ‘What are you, disgusting?’”

Some Trump aides say they think Trump is now in on the joke himself - at least with some senior officials.

In the Oval Office in 2017, Agriculture Secretary Sonny Perdue coughed in a meeting and Trump was “all grossed out, but it was more joking rather than angry,” said one former senior administration official with knowledge of the incident.

“He was like, ‘Ahh, you’re going to get me sick, move back’. He kind of yelled at him, like, ‘You’re going to get me sick!’”

Trump’s germaphobia can even help end intense internal arguments. After the 2016 campaign, Trump heard about a report that showed that the Trump campaign had paid almost US$94 million to digital guru Brad Parscale’s company Giles-Parscale.

Trump came down to the campaign headquarters on the 14th floor of Trump Tower to dress Parscale down, according to a campaign official familiar with the events and another person familiar with the episode. Trump demanded Parscale tell him where his money went, why so much money went through his account and how much of it he was keeping for himself.

David Bossie, who was deputy campaign manager in 2016, inserted himself on Parscale’s side and said that the money was mostly used to pay for digital advertising and was a standard part of a campaign. Kellyanne Conway, who had been campaign manager, made similar points but then she coughed - causing Trump to go “nuts” and soon leave the area, according to one of the people.

At that, the argument was over, shortened by the distraction of Conway’s cough.