Newcastle boss Steve Bruce said two of his players are "not well at all" after contracting coronavirus.
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.