I think it is true that we define sick differently, but I also think our definition of what is an ill child is coloured by our own circumstances. If it is convenient for us to keep the child home, we can easily say, "oh he is too ill for nursery", and if we have an important meeting, or a deadline, we may take a look at the child and say "sure, he is fit for nursery/school".
My friend had not read up on our school policy, and sent her son in after 24 hours (vomiting bug) and she got a call pronto to come and pick him up. When he arrived in, they had sent him straight to "the sick
bay", so away from the class.
My oldest son was looking a bit white in the face and complained that he felt a little nauseaus before the bell rang, and headmistress walked past, and she said I should take him home. I did. He did not have the bug, but it was better to err on the side of caution.
My youngest sons nursery seems to have no clear cut policies, and there is no information (one of the reasons I am taking him out of there and moving him to a new one). They are not informing about illnesses, I have spoken to parents who have confirmed to me that their child has a vomiting bug, CP, or hand foot mouth, but if you question the nursery, they dont know it. I have seen a child vomit on the premises, been told, oh, no there is no bug going, she is just choking on paint (!!!), or some other excuse.