The eyes are always moving. They’re looking up, down, left and right.

Secret Service agents in the presidential protective detail are on constant alert. Rooftops, a face in an open window, the surging crowd, the well-wisher on the rope line who’s reaching out an enthusiastic hand — all are minatory. The well-trained gazes keep scrutinizing, evaluating the possibility of a threat. Watching.

But despite such steely vigilance, the presidential protective detail these disquieting days is pretty much helpless as it attempts to fend off a new menace. It’s a devil — a potential silent assassin — that floats perniciously in the air, or in a perfunctory handshake, or even a riotous laugh. It’s invisible to the naked eye. You can’t shoot it. You can’t wrestle it to the ground and cuff it. You can’t at the last moment shove POTUS out of the way.

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The only shaky defense is common sense and wishful thinking. And that’s a quotidian strategy — one part makeshift to another part devil-may-care insouciance — that leaves President Trump Donald John TrumpObama calls on Senate not to fill Ginsburg's vacancy until after election Planned Parenthood: 'The fate of our rights' depends on Ginsburg replacement Progressive group to spend M in ad campaign on Supreme Court vacancy MORE as vulnerable as if he were driving down Dealey Plaza in Dallas in an open-topped limousine.

There’s a rogue’s gallery of photo ops documenting the unknowing yet nevertheless potential assassins — men who either have been infected or simply exposed to the coronavirus — who’ve slipped by in plain sight of the Secret Service and matter-of-factly gotten this close to the president: The Brazilian presidential press secretary standing shoulder-to-shoulder with a jack-o-lantern-grinning Trump holding a mud-brown “Make Brazil Great Again” cap as they celebrate the brand’s international launch; the Brazilian charge d’affaires who shared a festive dinner table with the president in the rococo splendor of Mar-a-Lago, statesmen fiddling away the evening while the virus burned its path across their nations.

And then there’s Matt Gaetz Matthew (Matt) GaetzLara Trump campaigns with far-right activist candidate Laura Loomer in Florida House to vote on removing cannabis from list of controlled substances The Hill's 12:30 Report: Sights and sounds from GOP convention night 1 MORE, a Florida Republican congressman who hitched a ride in the presidential limousine and then on Air Force One, only to be informed in mid-flight that he’d been exposed to someone who’d tested positive for the virus.

The list of possible carriers who get in chilling proximity to the president will, I predict, only keep growing disconcertingly — just like everything toxic associated with this pernicious virus.

So what’s the Secret Service to do?

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Well, there are shrewd lessons to be learned from their agency’s watch on Franklin Delano Roosevelt more than 75 years ago. Mike Reilly, the garrulous 33-year-old former college football player from Montana who took over the presidential security detail just days after the U.S. entered World War II, put his unique challenge in stark, decidedly politically incorrect terms: FDR “was a helpless cripple, incapable of walking a single foot.” He was, quite literally, a sitting target.

The president — in spite of his disability, in spite of his dodgy heart, full of a staunch courage — decided in November 1943 to travel halfway around the world in the midst of war to Tehran, to confer with the Soviet Union’s Josef Stalin and Britain’s Winston Churchill. It was the first time the three Allied leaders would be in one room.

Reilly would later write that he felt as unnerved as if “The Boss” (as he breezily called the president) had announced he was heading off to the Moon. He recovered enough to say, “There is grave risk, human beings and machines being what they are. But, if you feel you must go, we will make it as safe as we can.”

It was only when Reilly arrived in Tehran on a reconnaissance trip two days before the president’s arrival that he realized there was a lot more to worry about than “human beings and machines.” There was a hovering, seemingly ubiquitous invisible threat — disease. In the city, “everyone,” he would write, “obtained drinking water by scooping it from a stream that runs along the street gutter. This stream also serves as the city’s sewage disposal system. Typhus lice abound in great number, and typhoid fever takes a terrific toll of lives.”

And if all that were not a sufficient hazard to a president with a fragile immune system, when an already-spooked Reilly inspected the Iranian shah’s royal train that was to transport The Boss from Basra to Tehran, he discovered gold-plated cars with solid gold dining service — a train that was “certainly the finest typhus-laden piece of traveling equipment in the world.”

So Reilly went to work. Despite protests from FDR’s physician, he managed to get the president’s pilot to promise that he could skim over the mountaintops between Basra and Tehran without ever topping six thousand feet — an altitude that wouldn’t strain the weak hearts in the presidential party. It was risky, but then there’d be no need for the gilded yet virulent train. The pilot, Major Otis Bryan, skillfully did just what he said he could.

As for the danger of the city’s polluted water, Reilly breathed a sigh of relief when he discovered “the only buildings in Tehran supplied with running water are the American, British and Russian Legations.” Army engineers had inventively constructed water tunnels that delivered fresh mountain water to the diplomatic compounds.

Yet, as Reilly would soon learn, this was a decidedly mixed blessing: Nazi commandos had parachuted into Iran with the intention of traipsing through one of the concrete water tunnels to surprise and then assassinate FDR, Churchill and Stalin.

It was that unforeseen, unpredicted peril in Tehran that underscores the enormity of the challenge faced today by the Secret Service agents who inherited Mike Reilly’s job: You can plan, but you just don’t know what’s out there.

In the end, as a philosophical Winston Churchill wrote after his escape from an assassination attempt, the consequences are determined by “the inscrutable workings of fate.”

Today, too, as the coronavirus lurks ominously about the president’s world, all that his bodyguards ultimately can do is passively observe fate’s “inscrutable workings” ... and breathe a sigh of relief that Trump’s coronavirus test came back “negative” on Saturday night.

…and worry about the next time.

Howard Blum is a writer and contributing editor for Vanity Fair, a former Village Voice and New York Times reporter, and the author of more than a dozen nonfiction books. His most recent, “In the Enemy’s House: The Secret Saga of the FBI Agent and the Code Breaker Who Caught the Russian Spies,” was published in 2018. His next, “Night of the Assassins: The Untold Story of Hitler’s Plot to Kill FDR, Churchill and Stalin,” will be published in June by HarperCollins.