Newcastle boss Steve Bruce said two of his players are “not well at all” after contracting coronavirus.
An outbreak forced the club’s training ground to close for a week, while a game at Aston Villa was postponed.
Bruce has not named those who are still suffering, or how many tested positive.
“If you ever underestimated this thing then don’t, because you’re talking about elite professionals here and the way it got a hold of everybody was quite scary stuff,” said Bruce.
“For everybody out there, for goodness’ sake, stay safe. I know it’s Christmas but after what we’ve just witnessed then you’ve got to be careful.”
Bruce said that midfielder Isaac Hayden, 25, played in Saturday’s win over West Brom after recovering from coronavirus.
Meanwhile, Argentine defender Federico Fernandez, 31, revealed on Instagram on Sunday that he had recovered from the virus and he now could play against Leeds on Wednesday.
“He’s tested negative and I can say that because he himself revealed that he’s had the virus,” said Bruce. “But he’s OK now.
“[Players and staff have] had three negative tests in the past six days so it does look as if we’ve overcome it. But we’ve got two players in particular who are not well at all, and two members of staff who are still poorly.
“We’re relieved that nobody else has got it but we’re also concerned about the members of staff and two players who it’s having a bad effect on.”