A.,
I'm so sorry about your little one winding up in the hospital (been there, done that!) and that you have had to suffer through the evil RV (rotavirus) - been there, done that also!
I don't think there are specific laws requiring the center to tell about common viruses/illnesses (Strep throat is another very contagious pain in the butt!). Common sicknesses are not treated like communicable social diseases like lice, scabies, etc., where the center IS required by law inform everyone. However, a good daycare will inform parents, my old daycare used to have "the sick board" where they listed (in numbers) how many kids had what "type" of illness, like "Stomach flu", "Strep", etc. I knew whenever I saw the "Strep" number went up, I would probably be visiting the doctor. RV is a very nasty tummy bug, my daughter also wound up in the hospital at age 2 because of it. It's super contagious to the point where it is almost impossible to stop the spread. Most parents don't realize that RV is STILL contagious until the diarrhea is COMPLETELY gone, even if a child has just one runny BM a day - they are STILL contagious, or some parents DO know it's still contagious, but send their kids back to daycare anyway. This is how it spreads like wildfire - the child has stopped throwing up, they are eating normally, and they aren't having BM's all the time, but when they do go #2 - they still have diarrhea. Unfortunately, some parents send them back to daycare and their child just ends up passing it to another.
One point on the side of the daycare - some parents just don't inform the center of what/why their child has been out. Ultimately, if the parents don't inform the center, how can the center inform the other parents? I also think it is the parents' responsibility to keep their kids home until they are completely better to help control the spread.
I hope you both are feeling better very soon, and remember - there is NOTHING like a big spray bottle of LYSOL!
P.