If the president engages in risky behavior during the COVID-19 pandemic, can the Secret Service – or anyone – stop him?

Patricia Beckford Acheson can empathize with the Secret Service detail protecting President Donald Trump.

Acheson was among the first women assigned to the agency's permanent protection detail, spending more than three years protecting then-Vice President George H.W. Bush in the early 1980s.

"Our biggest fear was the lone assassin, but things have changed so much," the retired agent told USA TODAY. "Now they are dealing with organized terrorism and with COVID. The threats against a president nowadays are mind-boggling."

Agents make safety recommendations that generally are accepted. If not, such as in the case of presidents exiting limousines to greet crowds, some kind of mutual accommodations usually are made. The bottom line is that the president makes the call.

"Ultimately, our job is not to say 'no,' it is to protect," Acheson said.

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Former Secret Service directors W. Ralph Basham and Lewis Merletti said the White House medical office is responsible for providing advice and recommendations on the potential threat posed by disease and other health issues, which are factored into the overall security plan. Basham said the security and health challenge posed by the coronavirus is unprecedented.

“We are in uncharted waters,” said Basham, who served as director from 2003 to 2006. "We haven’t seen anything like this before."

Basham's sentiments were underscored Sunday, when Trump, accompanied by an elaborate Secret Service detail, briefly exited Walter Reed National Military Medical Center for a drive by flag-waving supporters outside at parade pace.

The impromptu public relations move required masked Secret Service agents to drive the infected president inside the close confines of an armored SUV.

Forget those made-for-TV moments when agents hustle a balking president into a bunker. They may push the president out of immediate harm's way, but longer-term decisions are subject to presidential approval. How well they are received often depends on the relationship between the president and his protective detail, former Secret Service director John Magaw said.

"You have to be careful because you don’t want to cry wolf, but you also want to be clear about any concerns you may have," Magaw said.

Trump invited dozens of politicians and high-profile dignitaries to the White House Rose Garden on Sept. 26 to introduce Amy Coney Barrett as his pick to succeed the late Justice Ruth Bader Ginsburg on the nation's high court. Many at the gathering did not wear masks, and social distancing was not apparent. Audience members sat in chairs packed next to each other, and some of them exchanged hugs. After that, the president and at least seven others tested positive for COVID-19.

The Secret Service declined to comment.

"I feel so badly because agents have a job, and they are sworn to do it and they will do it," Acheson said. "But how do you protect someone from COVID?"

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Trump took a three-month hiatus from large public rallies starting in March. That ended with a rally in Tulsa, Oklahoma, on June 20 that drew a far-from-capacity crowd to the 19,000-seat BOK Center. The New York Times, citing unnamed sources, said two Secret Service agents tested positive in Tulsa. Other campaign events featuring little or no social distancing followed.

“If he wants to do something, then we will do everything in our power to make sure that it can be done safely and securely," Magaw said. "The bottom line, it’s his call.”

Ogbonnaya Omenka, an assistant professor who specializes in public health at Butler University's college of Pharmacy and Health Sciences, told USA TODAY that in an "ideal world," such gatherings would not take place in the virus climate.

"For any public gathering, maintaining the public health guidelines should be a top priority," he said. "However, no matter how sound the public health recommendations are, the key consideration lies in whether organizers ... adhere to them or not."

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Though largely a responsibility for medical authorities, the virus could raise sensitive security questions for the Secret Service, Basham said. If the agency suspected, for example, that the president was ill with COVID-19 during a debate or other joint appearance with his Democratic rival, Basham said, the service would probably have the obligation to take its concerns to the White House medical office so Joe Biden – another service protectee – could be alerted to the risk.

“These are very difficult situations to be in,” Basham said.

The Secret Service was founded to combat widespread counterfeiting of U.S. currency after the Civil War. In 1901, the agency was asked to begin its protective mission after the assassination of President William McKinley.

Less than two decades later, the "Spanish flu" emerged in 1918. Much like Trump, President Woodrow Wilson downplayed it. And like Trump, Wilson became infected, contracting the illness shortly after arriving in Paris in April 1919. Sarah Fling, a fellow writing for the White House Historical Association, said a number of members of the Wilson entourage became ill during a trans-Atlantic voyage in February 1919 – including several members of the Secret Service.

The Secret Service doesn't protect presidents alone, calling upon other federal, state and local agencies to assist on a daily basis. When the president travels, an advance team of Secret Service agents works with the host city, state and local law enforcement and public safety officials to put the necessary security measures in place.

When Bush plunged into a friendly crowd, Magaw said, he often had a hand on the president's belt, so he could physically move him in the event of an emergency. He said Bush knew of the need for hands-on protection and accommodated the service by wearing uniformly tailored suits with a single belt loop on the back of his pants, so Magaw had a relatively clear spot to grasp without becoming entangled.

The agency was moved to act when Bush's penchant for speedboating on the waters near his family retreat in Maine raised concerns. Magaw said he talked to Bush about installing a safety bar near the captain's chair, where the president and Secret Service agent would attach themselves to guard against an unexpected fall overboard.

“The relationship and rapport is very important,” Magaw said. “That’s why you spend time with them and talk through exactly how you are going to proceed."

This article originally appeared on USA TODAY: Trump COVID-19: Can Secret Service stop president's risky behavior?