The Prime Minister today revealed he had tested positive for coronavirus after experiencing mild symptoms yesterday.
With Mr Johnson now self-isolating, Downing Street must work out all the people he has had contact with who could now be at risk.
Multiple No10 staff, the Chancellor and possibly Matt Hancock the Health Secretary, could also have to self-isolate if he has spread it to them.
Downing Street said this morning that they did not need to do so unless they develop symptoms.
Cabinet meetings have been taking place digitally this week in a bid to prevent the virus spreading by staying home.
Despite this, both Mr Hancock and Cabinet Secretary Mark Sedwill both attended Tuesday’s meeting in person.
Last night he appeared alongside the Chancellor Rishi Sunak as the pair applauded as part of the “Clap for Carers” campaign outside Downing Street.
Mr Sunak is not showing symptoms, and has therefore not been tested for the disease.
Noticing symptoms yesterday, just one day earlier the PM had sat in close proximity to a series of his cabinet colleagues at Prime Ministers Questions.
The 55-year-old sat next to Home Secretary Priti Patel, with many other MPs attending the event.
He has also been regularly meeting his crack team of Chief Medical Officer Chris Whitty and Chief Scientific Adviser Patrick Vallance.
On March 20, he met with SNP Westminster leader Ian Blackford to discuss the economic impact of coronavirus.
Amid his busy schedule, Boris normally sees the Queen for their regular audience every week.
However, the PM has not seen Her Majesty in person for more than two weeks.
The PM announced the diagnosis today with a video released on his Twitter account.
He explained: “I have developed minor symptoms of the coronavirus, a temperature and a persistent cough and on the advice of the chief medical officer I have taken a test. That has come out positive.
“I am working from home, I am self-isolating, and that is entirely the right thing to do.
“But be in no doubt that I can continue thanks to the wizardry of modern technology to communicate with all my top team to lead the national fightback against coronavirus.
“I want to thank above all our NHS staff.
“It was very moving last night to join the national clap, but it’s not just the NHS, it’s our police, our social care workers, teachers, everybody who works in schools, DWP staff.”
He will have to self-isolate from his girlfriend and fiance, Carrie Symonds, who is currently pregnant – and in the more at risk category.
The news comes just weeks after Boris said that he was continuing to shake hands with everyone, despite the risk of the virus spreading.
It may mean that Dominic Raab, the Foreign Secretary, will have to take over duties if the PM becomes more ill.