He was the first leader of a major world power to test positive for coronavirus and was admitted to hospital on Sunday evening (local time) with a persistent fever and cough.

Experts said his move to the intensive care unit meant the UK leader was “extremely sick”, but a spokesman said Johnson was “conscious” and the move was a precaution.

Here’s what we know.

When did he get coronavirus?

Johnson tested positive to COVID-19 on March 26, releasing a short video on his Twitter account saying he had “developed mild symptoms” in the previous 24 hours.

He started self-isolating and was working from home and held the UK’s first digital Cabinet meeting.

After posting regular video updates to social media, Johnson was admitted to St Thomas’ Hospital in central London on Sunday evening (local time) on the advice of his doctors.

What are his symptoms?

Johnson was initially tested in hospital after displaying a high temperature and a cough, according to the BBC.

A spokesman said these symptoms were persisting.

What are his symptoms?

Johnson was initially tested in hospital after displaying a high temperature and a cough, according to the BBC.

A spokesman said these symptoms were persisting.

How sick is he?

Downing Street said Johnson was admitted to ICU “on the advice of his medical team” after his symptoms “worsened”.

A spokesman said he was “conscious at this time” and had been moved there “as a precaution should he require ventilation to aid his recovery”.

Medical experts said it was a serious situation.

“There is no doubt this turn of events means Boris Johnson is extremely sick,” Derek Hill, a professor of medical imaging at University College London, said.

“One of the features of COVID-19 in all countries seems to be that many more men become seriously ill than women — especially in the over 40 age group.

“Also, we know that people under about 60 seem to have a higher chance of making a recovery from critical illness with COVID-19 than older people.”

Linda Bauld, a professor of public health at the University of Edinburgh, said: “The admission of the Prime Minister to intensive care is of huge concern and illustrates just how indiscriminate this virus is.

“Anyone anywhere, including the most privileged in our society, can be affected and can become seriously ill.

“It is imperative now, more than ever that the rest of us comply with government guidelines to stay at home and not put others at risk.”

What do we know about the hospital?

St Thomas’ is a well-known public teaching hospital across the river from Westminster and the Houses of Parliament.

It is a teaching hospital and has “one of the largest critical care units in the UK and one of the busiest emergency departments in London”.

It has been linked with several famous names in medicine, including nurse Florence Nightingale.

Former prime minister David Cameron said Johnson was “in great hands”.

According to Downing Street, Johnson has asked Foreign Secretary Dominic Raab to deputise “where necessary”.

Raab holds the honorary title of First Secretary of State — the de facto “second in charge” of the Government.

The 46-year-old was a former Brexit Secretary and ran for the Conservative Party leadership in 2019 before going on to endorse Johnson.

What happens next?

Raab said, “the Government’s business will continue”.

“The Prime Minister is in safe hands … and the focus of the Government will continue to be on making sure that the Prime Minister’s direction, all the plans for making sure that we can defeat coronavirus and can pull the country through this challenge, will be taken forward,” he said.

He will chair the daily Cabinet meeting with medical and scientific experts and “take responsibility on matters like security and foreign affairs”, according to the BBC.

The BBC reported that Raab will “have to make the big decisions”, including financial, about the UK Government’s reaction to the coronavirus crisis while the Prime Minister cannot.

– ABC / Liam Butterworth