(The Wall Street Journal) LONDON – In a side room of St Thomas’ Hospital in central London, doctors faced a decision. Prime Minister Boris Johnson lay sick in the hospital’s east wing, struggling to get enough oxygen. Doctors were thinking of putting him on a ventilator, a step that is often a last resort for Covid-19 patients.

Mr. Johnson grew agitated. Avoid using a ventilator if at all possible, he told his doctors.

He worried that even if he shook off the disease, the process of sedation and inserting a tube into his lungs would force an extended recovery, said a person familiar with his care. That would leave the leader of Britain sidelined while a pandemic raged.

Mr. Johnson is back at Downing Street now, after a hospital stay that included three nights in intensive care, ultimately without mechanical help for his breathing.

A reconstruction of his battle with the coronavirus, based on interviews with officials, friends of Mr. Johnson and people familiar with his care, offers a glimpse into the strange challenge of leading a national response to a pandemic while floored by it – all in a country with no fixed succession plan for its political leaders. Mr. Johnson is the highest-profile world leader known to have contracted Covid-19.

Also read: SLR: “Loved and loathed” Boris Johnson convalescing to face the music on Covid-19 handling

Mr. Johnson, 55 years old, has been chastened by the experience, say people who know him. They say his illness has left him feeling occasionally tired and made him more cautious about how the government should go about eventually lifting a lockdown. He’s trying to lose weight, concerned that his physical condition may be partly to blame for the severity of his sickness.

A spokesman for Mr. Johnson declined to comment.

As he recovered last month in the prime minister’s 16th-century country residence in southern England, a number of cabinet members joined him on a video conference. “Salus populi suprema lex esto,” said Mr. Johnson, a student of Latin and Greek, quoting the Roman politician and philosopher Cicero. Let the health of the people be the highest law.

Known for his unruly blond hair and championing of Brexit, Mr. Johnson has throughout his career rebounded from setbacks, using an optimistic, humorous style to flip awkward situations to his advantage. Early in the pandemic, he took the virus seriously but acted as if he were personally impervious, aides say.

In early March, he said he visited a hospital that held coronavirus patients and glad-handed those around him. He attended a rugby match in a crowded stadium. He talked to aides about his reluctance to shut people in their homes.

When he unveiled social-distancing recommendations, he did so standing close to two others. He continued attending cabinet meetings in person while others dialled in.

By late March, an adviser who leads Brexit trade negotiations was self-isolating with symptoms. A cluster of Mr. Johnson’s other aides, including his chief of staff and chief medical officer, got sick as well. Mr. Johnson was working in a warren of buildings around Downing Street with a labyrinth of tight corridors and small rooms, well suited to be a viral vector.

On March 26, Mr. Johnson stood outside Downing Street to join a nationwide applause for hospital workers, but he was feeling unwell. Britain’s chief medical officer had advised him to get a test for Covid-19. At midnight he received the result: positive.

Mr. Johnson holed up in his apartment at 11 Downing Street. He had the use of a bedroom with an adjoining bathroom and a study lined with red-leather-bound books, traditionally used by the Chancellor of the Exchequer. Meals and briefing papers were left at his door.

When a friend texted to check on him, he seemed to brush off the virus, saying his focus was “sorting out the country.”

Mr. Johnson often returned to Cicero during his illness. Explaining why he wanted to continue working, he cited advice Cicero was given by his brother Quintus about the political importance of remaining in Rome and staying visible to win public support.

Despite feeling what one person described as “floppy,” Mr. Johnson plowed on. The day after his diagnosis, he called President Trump, who later said Mr. Johnson “didn’t sound good at all.”

“Before he even said hello he said, ‘We need ventilators,’” Mr. Trump said.

Mr. Johnson led coronavirus response meetings by video, where he was seen coughing. He looked increasingly unkempt and pale. Some colleagues started to fret that he wasn’t getting better.

In Britain, there is no private physician for the prime minister. The leader must rely on the National Health Service.

A week after his diagnosis, although Mr. Johnson wasn’t gasping to breathe, his doctors grew concerned that the level of oxygen saturation in his blood was too low.

His fiancée, Carrie Symonds, pregnant and herself ill with symptoms of Covid-19, urged Mr. Johnson to get further treatment. Doctors, officials and Ms. Symonds eventually persuaded him to go to a hospital. She told him she needed him cured to help with the imminent arrival of their child.

At 8pm on Sunday, April 5, as Queen Elizabeth rallied the nation with a televised address, an aide drove Mr. Johnson south across the Thames to St Thomas’ hospital. Inside the concrete tower he was placed on a general ward on the 12th floor and given oxygen through a tube in his nose, and then through a mask.

At the hospital, “the Prime Minister was treated with the same level of care as any of our patients, and appropriate security measures were put in place,” a spokeswoman for St Thomas’ said.

Still, there were a few special efforts, including keeping a side room and equipment on standby for intensive care, said a person familiar with his care, who added that medics believed he should have come sooner.

His doctors worried about what they call a 10-day crash, where in the second week of their illness, some Covid-19 patients rapidly worsen.

On Monday, April 6, which was 11 days after Mr. Johnson’s positive test, his condition deteriorated. A monitor showed his blood oxygen slipping. Within hours, the situation became grave and the ICU prepared for his arrival.

He had aching muscles, a sign of oxygen starvation, which can cause rapid damage to the body. But ventilation can result in harm, too, with the harsh process of forcing oxygen into a the lungs and the heavy sedation required.

At 7 p.m. that Monday, orderlies wheeled Mr. Johnson to a side room in the ICU in the east wing. Physicians Nicholas Price and Nicholas Hart, experts in respiratory disease, led his care.

In Westminster, a constitutional headache arose. Britain’s system doesn’t spell out what happens if a prime minister dies. Mr. Johnson’s Conservative Party has no protocol for replacing a deceased leader. If the prime minister died, party bylaws would have to be rewritten to speed the often weeks-long process of picking a new leader.

Mr. Johnson last year named Foreign Secretary Dominic Raab as his stand-in when he is absent. The queen didn’t hold her traditional weekly audience with the country’s leader, awaiting instead Mr. Johnson’s return.

The situation took a surreal diplomatic turn after President Trump said he was putting a group of pharmaceutical companies in touch with doctors at St Thomas’ hospital to provide possible drugs to help Mr. Johnson. Among the companies, said a person familiar with the matter, was Gilead Sciences, maker of the experimental ebola drug remdesivir, which has shown some benefit against Covid-19. A representative of the company contacted the hospital to offer assistance, according to an official at St Thomas’.

The result was awkwardness at Downing Street, because Mr. Johnson couldn’t be seen getting drugs not available to all Britons. Hospital officials wouldn’t say whether the drug was administered to Mr. Johnson.

Mr. Johnson’s political team struggled with what to tell the public. His aides say they understood Mr. Johnson was getting briefing documents after being admitted to the hospital. They presented his move to the ICU as precautionary. A senior Conservative Party official said at no point was he told Mr. Johnson was close to death.

In the hospital, however, some feared the worst. Doctors late on Monday April 6 debated whether they needed to put Mr. Johnson on a ventilator, a discussion that continued until early the next morning.

Mr. Johnson’s blood-oxygen level fell below the 94% minimum regarded as safe, but not to below 80% where the functions of organs could be compromised, according to a person who received contemporaneous accounts of his progress. He received litres of oxygen.

In the event, by Tuesday, Mr. Johnson’s situation seemed more stable. His breathing became less laboured. His oxygen level improved, and doctors reduced the amount fed through his mask. On Thursday they released him into a ward to recuperate. Nurses and caregivers clapped.

Within hours of being wheeled out of intensive care, Mr. Johnson was standing in his boxer shorts by his hospital bed with two nurses to join a nationwide clap for medics.

On April 12, Mr. Johnson was discharged.

Guto Harri, an adviser when Mr. Johnson was mayor of London, said being hospitalised with Covid-19 would be a disaster for most leaders, “but it hasn’t hurt him at all. It has helped turn him into a symbol of the nation somehow.”

Others said his ordeal symbolised his government’s excessively relaxed management of the crisis when the virus first took hold. Britain is on track to have one of the highest death tolls from the virus in Europe.

“If he hadn’t been in intensive care, I think a lot more people would be questioning his handling of the crisis,” said Tim Bale,  professor of politics at Queen Mary University of London.

In a video after his release, Mr. Johnson thanked his caregivers—“men and women, but for some reason many of them called Nick.”

Mr. Johnson rejoined Ms. Symonds at Chequers, the countryside retreat, before returning to Downing Street on April 26.

A few days later, Ms. Symonds gave birth to a baby boy. They named him Wilfred Lawrie Nicholas: Wilfred for Mr. Johnson’s grandfather and Lawrie for Ms. Symonds’ grandfather. Nicholas, she said, was for the two doctors who had saved the prime minister’s life.

– Write to Max Colchester at [email protected] and Anna Isaac at [email protected].