Footballers have been warned spitting on the pitch during games could raise the risk of spreading swine flu.
Blackburn and Bolton both have players with signs of the virus ahead of their Carling Cup matches this week.
The Health Protection Agency said the habit of spitting "could increase the risk of passing on infections".
However, Chelsea coach Carlo Ancelotti said he was "not worried" about the threat of swine flu at his club despite having played Rovers on Saturday.
Blackburn boss Sam Allardyce suggested members of the Rovers squad, three of whom were confirmed with the virus earlier this week, may have passed on the infection during their 5-0 defeat by Chelsea last Saturday.
I know what to do if I have the flu. It's my grandmother's prescription - hot milk. Alcohol - red wine. Fantastic
But Ancelotti stated: "The flu is not only on the pitch, it's everywhere. We take all the precautions and we want to think about playing, that's it.
"We are not worried about this problem because we've taken all the precautions.
"I know very well what I have to do if I have the flu. It's not a medical prescription. It's my grandmother's prescription - hot milk. Alcohol - red wine. Fantastic."
Chelsea have told BBC Sport their medical department are taking precautions to ensure the health and well-being of the players.
The HPA's advice warns footballers against spitting, and looks to them as role models for the public to follow.
"Spitting is disgusting at all times. It's unhygienic and unhealthy, particularly if you spit close to other people," said an HPA spokesman.
"Footballers, like the rest of us, wouldn't spit indoors so they shouldn't do it on the football pitch.
"If they are spitting near other people it could certainly increase the risk of passing on infections.
"It's about setting examples for young people who idolise them."
But Ancelotti said he expected other, familiar practices on the field, such as swapping shirts, to carry on.
"We will do the same things we normally do," added the Italian coach.
"It's not a problem. I think the players have stayed very well up to this moment."
Meanwhile, Bolton, who play Chelsea in the Carling Cup fourth round on Wednesday, have revealed four of their players had the virus.
Asked if he was worried about his players coming into contact with Chelsea's, Bolton manager Gary Megson said: "The medical people say it is very difficult, if not impossible, to contract it in that way.
"You are going to get one or two with swine flu and I think on the playing staff I think we have had five, four players and one member of staff.
"They are all okay at the moment, but it wouldn't be right to start giving you names."
Manchester United, who host Blackburn on Saturday, refused to comment, while Rovers' Carling Cup game at Peterborough will go ahead on Tuesday.
Premier League players soon get over it because they are fit and healthy
Manchester City manager Mark Hughes
Earlier, Allardyce said Chelsea could be the next club to be affected by swine flu.
"There is no doubt about that," stated Allardyce.
"When you look at the medical side of it, it is one of the most infectious viruses we have had."
However, Manchester City manager Mark Hughes said most Premier League stars would recover from the virus quickly.
"What you find with Premier League clubs is that there are a lot of young, fit, energetic guys," said Hughes, whose defender Micah Richards contracted swine flu while on holiday in Cyprus in the summer.
"If they do get it, they are only laid low for a couple of days. They soon get over it because they are fit and healthy and are not compromised too much.
The Premier League says clubs should seek advice from the "relevant statutory" authorities regarding the virus, but Rovers boss Allardyce has questioned the wisdom of letting Saturday's match at Stamford Bridge proceed, after Blackburn informed them of their first two cases of swine flu.
The Premier League have also told BBC Sport that they do not anticipate at this stage a situation where any game will be postponed as a result of swine flu.
But Allardyce believes the Premier League are shirking responsibility.
He said: "They know that we went through the right procedures and that we did not call it off because we couldn't."
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