Scene One: a brunch party for thirty-somethings. The well organised hostess has laid on a meal for the kiddies while their parents booze in a room nearby.

But one small girl has a runny nose, and the mother of a little boy insists that her child take his meal in an upstairs room, for fear of infection. Quietly, she tells various guests how shocked she is that the little girl's parents brought her to the party.

The parents of the little girl are not amused, when word reaches them, and are heard muttering words like "neurotic" and "paranoid" as they head for home.

Scene two: another daytime party, more parents grazing and boozing, another gaggle of kids. Two, it seems, are pretty sick: they're not just sniffling, they've got vomiting and diarrhoea. One mother spots this, but doesn't make a scene. She takes her children aside quietly and says: "When you eat, wash your hands."

Judging when your child is sick - sick enough to keep her home from the creche, away from parties, away from other children - is a difficult decision for most parents. The question of what to do when another parent brings a really sick child into contact with yours - or accuses you of doing so - is one demanding great social diplomacy.

It's unlikely you'll find the answer in any book on etiquette.

But increasingly, with more and more very young babies and children being minded by childminders and in creches, it's an issue parents have to tackle.

And it's surprising how different parents' attitudes to sickness can be, among adults who would appear to have similar education and concern about their children's welfare.

Some of them panic at the slightest sniffle, race to the doctor, heat up the chicken soup, don their Florence Nightingale outfit and tuck the child up in bed. Others have the same tough-it-out attitude that they take towards their own sickness and wait patiently to see how symptoms develop.

A mild sniffle for one parent will be borderline pneumonia to the more neurotic; and, inevitably, parents will find themselves shocked, even annoyed at the "irresponsible" or the "paranoid" behaviour of others.

What to do? Dr Madeleine McCarthy, a GP in Dun Laoghaire, Co Dublin, believes that all parents should be responsible if they think their child has an infectious disease - but should use their common sense as well.

"The responsibility for controlling infectious diseases lies with all parents, and the obvious first thing to do is to immunise your children against measles, mumps and rubella as well, of course, against diseases like whooping cough. After that, common sense comes into it."

McCarthy agrees that it can be difficult for parents to know when to keep a sick child away from other children - it's true that the infectious stage of an illness is actually often passing just about the time the illness is diagnosed.

The diseases she would worry about are chicken pox, German measles, the odd case of mumps, and scarlet fever, a form of streptococcal throat infection.

"Usually, where a GP would diagnose an infectious illness, he or she would give clear guidelines on what to do."

After that, the best hope is that all parents will behave as responsibly as they can by not, for example, bringing a quite sick child to a creche - though she readily confirms that "it's not unknown for parents to put children into creches when they're not quite well".

Parents should try not to expose children who are less than a year old to too much risk of infection; in an ideal world, McCarthy reckons, parents wouldn't put such children into a creche until they've passed their first birthday.

That said, children can't live in a plastic bubble "and the bottom line is that they will get sick at some stage, whether it's in a creche, playschool or big school. It's just that illness is generally easier to deal with, and less dangerous, in an older child.

"But you're doing your child no favours by isolating him or her from another child who has a cold, because then your children would build up no natural resistance, no immunity."

McCarthy would also agree with some parents who would deliberately allow their child to be exposed to a disease like chicken pox, if they reckon the child is healthy enough to cope with it - after all, there is no vaccine against it, it is contagious and it's much more serious and painful to get it as an adult.

What about the party scene? "Well, you shouldn't bring your child into a social situation if he or she is unwell - that is, has a fever, or vomiting and diarrhoea. A runny nose doesn't qualify."

And what if you do find that another parent has brought an infectiously ill child to the party? That's a tricky one, McCarthy says, but she would agree that quietly advising your own child to wash her hands would be sensible. Or perhaps you might quietly make your excuses and leave without making a fuss.