What would happen if the president was put on a ventilator?

The hospitalization of British Prime Minister Boris Johnson, and his subsequent move to intensive care to receive supplemental oxygen, marked the first known case of a world leader seriously affected by the coronavirus.

President Donald Trump said he has been tested twice for the virus, once in March and again last week. But what if a president had to be hospitalized and put on a ventilator?

Dr. John Torres, an NBC News medical correspondent, explained that when patients need a ventilator, a breathing tube must be inserted first. "That requires a patient to be sedated, effectively incapacitated. Otherwise it's not possible to intubate them."

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Who would take over running the country then? The answer comes from the Constitution's 25th Amendment.

It provides that whenever the president formally notifies Congress that "he is unable to discharge the powers and duties of his office," the vice president becomes the acting president until the president notifies Congress he is again fit for office. The provision thus allows the president to transfer his authority to the vice president for short periods of time.

That particular authority been exercised three times since the amendment was adopted in 1967, and in each case the president knew in advance that a medical procedure would require anesthesia. It was informally invoked by Ronald Reagan when he had colon cancer surgery and twice by George W. Bush for colonoscopy.

If the president suddenly becomes incapacitated and is unable to transmit notice to Congress, the amendment provides that the vice president becomes acting president, once the vice president and a majority of Cabinet members notify Congress that the president cannot carry out his duties.

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This provision has never been used, even after President Reagan was shot in 1981. White House senior staff members discussed invoking the amendment, but Vice President George H.W. Bush was traveling. By the time he returned to Washington, the president's condition had stabilized and his doctors anticipated a full recovery.

While the amendment says a decision that a president is incapacitated must have the concurrence of either a majority of the Cabinet or "such other body as Congress may by law provide," Congress has never adopted legislation to provide for a presidential disability review group, though several proposals have been considered.