THUS THE RACIAL HYGIENE OBSESSION:

He asks visitors if they'd like to wash their hands in a bathroom near the Oval Office.

He'll send a military doctor to help an aide caught coughing on Air Force One.

And the first thing he often tells his body man upon entering the Beast after shaking countless hands at campaign events: "Give me the stuff" -- an immediate squirt of Purell.

Two and a half years into his term, President Donald Trump is solidifying his standing as the most germ-conscious man to ever lead the free world. 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."

Hitler seemed obsessed with the idea of infection. The Nazi leader was, by most accounts, a germaphobe who avoided personal contact and bathed incessantly. He was repelled by sex, horrified by venereal disease. He referred to himself as an Einsiedler - a hermit. He extolled the virtues of celibacy and claimed prostitution was for inferior races, though some have proposed Hitler himself contracted syphilis from a Jewish prostitute in Vienna in 1908. 

It was in ideology, however, where Hitler's obsession with infection thrived, becoming the essential Nazi metaphor: Germany was the body, Jews were the parasites. 

Examples are abundant in his speeches and writings:

"How many diseases have their origin in the Jewish virus! We shall regain our health only be eliminating the Jew."

"Anyone who wants to cure this era, which is inwardly sick and rotten, must first of all summon up the courage to make clear the causes of this disease."

"This is the battle against a veritable world sickness which threatens to infect the peoples, a plague that devastates whole peoples...an international pestilence."

"The Jew is a parasite in the body of other nations."

"Germany, without blinking an eyelid, for whole decades admitted these Jews by the hundred thousand. But now... when the nation is no longer willing to be sucked dry by these parasites, on every side one hears nothing but laments."

"If this battle should not come...Germany would decay and at best sink to ruin like a rotting corpse."

Do Hitler's germaphobic tendencies and obsession with the infection metaphor reveal anything about his personality traits? While it's impossible to know for sure, it seems likely that he was highly sensitive to disgust.

Over the past couple of decades, studies have linked disgust sensitivity to numerous dimensions of ideology - immigration, political affiliation, sense of justice. If Hitler ranked high on the disgust scale, there were probably deeply rooted psychological forces lurking underneath his xenophobia and murderous fantasies that research on the behavioral immune system might help bring to light.

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