Maybe they should have made the parents aware, MrsGrady - but what would the OP have done with the information?
Unless she was willing to take her child out of nursery every time another child might be ill, or she might be in contact with infection, then just knowing about the ill child isn't going to reduce or remove the chance of her child becoming ill.
I would expect the nursery to have policies to try to reduce the risks of infection and cross infection - sending children home if they have a communicable disease, good hygiene practices - that sort of thing - but I imagine it is not always easy to enforce them - if a child seems well when dropped off, but becomes ill, or if the parents ignore signs of illness because they can't or won't take time off work (parents knowingly send sick children to school - I wouldn't be surprised if sick children were sent in to nursery). If this happens, I assume they'd call the parents to pick the child up, and to to keep the child apart from the other children in the meantime - but staffing levels may be tight, and it may be hard to find enough staff to isolate a child.
Add to that the fact that some illnesses are contagious before the symptoms emerge, and I can see why it is very hard for nurseries to stop illness running through the place. They should try, of course, and they should keep parents informed, so they know to look out for symptoms, but they are on,y human.