Footballers were warned on Tuesday spitting could increase the risk of catching swine flu as Bolton joined English Premier League rivals Blackburn in saying squad members had contracted the virus.
England's Health Protection Agency (HPA) said the "disgusting" habit - a common sight at football matches - could see the infection passed on.
Blackburn and Bolton are set to be without several players for their League Cup matches this week because of swine flu.
"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.
"Certainly, spitting is a nasty habit that should be discouraged - and it should be discouraged by the clubs.
"It's about setting examples for young people who idolise them."
Bolton manager Gary Megson said Tuesday several players were suffering with swine flu. His comments came a day after Blackburn boss Sam Allardyce said the virus had struck at his club.
Bolton play Chelsea in the League Cup on Wednesday and while Allardyce was angry with officials for not giving a lead in getting games called off, Megson said: "From our own point of view, you're not going to be a business with quite a few employees and it totally escapes you."
Chelsea beat Blackburn 5-0 last weekend, a victory soon overshadowed by swine flu concerns.
However, the Premier League leaders' manager, Carlo Ancelotti, was relaxed about the prospect of members of his squad contracting the virus.
The Italian, informed by Rovers the day before the match of their swine flu problems, said Tuesday: "I'm not worried. 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.
"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."
An annoyed Allardyce said Blackburn had no option but to play Chelsea if they were to avoid punishment for calling the game off.
"The Premier League has tried to sit on the fence by saying that we didn't request to have the game called off," Allardyce said on Monday.
"We knew we couldn't get the game called off because we were told way back in February or March when we took some guidelines that unless there was some government legislation, everybody had to do whatever they had to do.
"If there was a swine flu outbreak, you couldn't call a game off, no matter what," he added.
"We fulfilled our total, 100 percent, responsibility by confirming two cases of swine flu and suggesting it looks like one or two other players and members of staff had similar symptoms, and we made the same phone call to Chelsea."
The attitude of football authorities towards swine flu has not been the same in all countries.
Last weekend's French League match between Marseille and Paris Saint-Germain was postponed after three players from the capital club contracted swine flu.