I have no immunity to Parvovirus either and they are most infectious before they show symptoms. I had my nephew staying with us for a week and he came down with a fever and vomiting, we took him to see the Dr and he said that mothers with more than 1 child will generally have to deal witha sick child at some point during thier PG and they are ok the majority of the time.
My only suggestion is to avoid unessessary contact with sick children and make sure you wash you hands after handling someone else child.
I work as a nurse and obviously have to be in close contact with sick people, I also needed to be treated for menigicoccal during the week I conceived as I had nursed a very sick patient and it was only the next day we found out why she was so sick. Luckliy I knew there was a chance I could be PG so made sure the meds were safe in PG (the recommended one isn't).
I hope this information reassures you a little.