Chelsea's full squad have gone into self-isolation after winger Callum Hudson-Odoi tested positive to the coronavirus, the club said in a statement.
The Premier League looks certain to be suspended now after Everton's entire first-team squad and coaching staff are in self-isolation after a player reported symptoms consistent with coronavirus.
Hudson-Odoi displayed cold symptoms on Monday morning and has stayed away from the club since then. His test result was returned on Thursday night.
"Chelsea men's team player Callum Hudson-Odoi had a positive test result for Coronavirus returned this evening," the Premier League club's statement said.
"Chelsea personnel who had recent close contact with the player in the men's team building will now self-isolate in line with Government health guidelines.
"These will include initially the full men's team squad, coaching staff and a number of backroom staff."
The club said despite testing positive, the 19-year-old was "doing well and looking forward to returning to the training ground as soon as it is possible".
In a video message on Hudson-Odoi's official Twitter feed, he said: "Hi guys. As you may be aware, I had the virus for the last couple of days, which I've recovered from.
Thank you for all your good wishes, as I said in the video I'm feeling good and hope to see you all soon!!🙏🏾❤️ pic.twitter.com/hn5d0ikm60— Callum Hudson-Odoi (@Calteck10) March 13, 2020
"I'm following the health guidelines and self-isolating myself from everybody for the week. I hope to see everybody soon, and hopefully be back on the pitch very soon. Take care."
The Blues have also closed two buildings at their training facility.
Chelsea are scheduled to play at Aston Villa on Saturday afternoon but the match looks unlikely to proceed.
The remainder of the Premier League season, in fact, is in doubt with the League to hold a meeting on Friday to determine a course of action in response to the coronavirus.
Meanwhile Everton have followed guidelines and players and staff have been told to remain at home with the training ground, Goodison Park and corporate headquarters at the Royal Liver Building all closed.
"The club is in regular contact with the player in question and is monitoring the wellbeing of all players and staff," said a club statement.
"The health, welfare and safety of all Everton players, staff and stakeholders is the club's absolute priority.
"Therefore, the club has taken the decision to close down all Everton workplaces including USM Finch Farm, Goodison Park and its headquarters at the Royal Liver Building in Liverpool City Centre.
"The club's retail store at Goodison Park will also close but its Liverpool ONE store is unaffected and will remain open."