Gareth Southgate reportedly tested positive for coronavirus last month but battled the illness in secret from his England players.
Southgate, 50, is believed to have developed symptoms in late October and subsequently returned a positive Covid-19 test.
He then spent 10 days in self-isolation before testing negative and returning to his duties ahead of the friendly with Republic of Ireland on Thursday and the upcoming Nations League clashes with Belgium and Iceland.
England players were not informed about Southgate's diagnosis, while he was unable to attend Premier League matches due to his term in self-isolation in his home in North Yorkshire.
There were heightened concerns about Southgate's wellbeing on account of his age, according to The Sun , but his fitness helped him fend off coronavirus.
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Southgate is known to regularly run and work out, and the report adds a source said: “He’s a professional, he just got on with it. Fortunately, he’s very fit for his age.”
Southgate is believed to have first tested positive on around 25 October, while he returned a negative test on 5 November.
This allowed him to announce his upcoming squad via Zoom and he has since returned multiple negative tests.
Southgate, alongside England's players and other personnel, is regularly being checked in line with the FA's protocols.
The former Middlesbrough boss has been a vocal supporter of the NHS throughout the pandemic and called on fans to adhere to lockdown rules in March.
Southgate said: "Life is very different for us all at the moment and sport rightfully stands aside to support a more important cause during these unprecedented times."
As well as agreeing to a 30 per cent pay cut on his £5million-per-year salary, Southgate called two key workers to personally thank them for their efforts.
He added: “It’s important the nation gets behind those playing their part in helping to return society to a sense of normality.”