Manchester City duo Craig Bellamy and Vladimir Weiss have contracted swine flu, with several other players under observation for the virus.
Weiss was ordered to stay away from Eastlands ahead of Saturday's Premier League victory over Chelsea after it was discovered he had been hit by the illness.
City have confirmed that Wales international Bellamy has also been suffering from the H1N1 virus, while a number of other staff members have reported flu-like symptoms and have had swabs taken.
"There is a little bit of it creeping into the camp," City assistant boss Mark Bowen told the Manchester Evening News.
"Players come in every day with sniffles, coughs and colds and the doctor has to determine whether it is the onset of something more serious.
"It is his decision whether they have to be sent home."
Bellamy is thought to be over the worst, although he is still doubtful for Saturday's short trip to Bolton as he is battling to recover from a knee injury.
One player who definitely will not be involved at the Reebok Stadium is England defender Wayne Bridge.
The former Chelsea star was carried off at the weekend following a clash with Juliano Belletti.
Manager Mark Hughes initially felt Bridge would be out of action for a couple of weeks.
However, scans have revealed the £12million full-back suffered medial ligament damage, which could sideline him for between four and six weeks.
That would mean Bridge must sit out both legs of City's Carling Cup semi-final with Manchester United, plus the Blues' FA Cup third-round tie at Middlesbrough.
In addition, City have eight league games to play before the end of January, although Bowen is hoping to get Bridge back slightly earlier than expected.
"It depends on his recovery but Wayne tells us he is a good healer and he is a good professional, so we are hoping it will be sooner," he said.