Rotovirus can get pretty nasty.  Newborns get dehydrated faster than bigger
kids.  The breastfeeding will be somewhat protective, but the baby is still at
risk for developing Rotovirus, no matter what the mom does.  However, mom
should take all precautions to try to keep the newborn from contracting
Rotovirus (which is spread primarily by the fecal-oral route):  i.e., keeping
the toddler apart from the infant as much as possible, compulsive handwashing
of her own hands and the toddler's hands, and breastfeeding as much as
possible.  If the baby starts to develop vomiting, fever, and diarrhea, he/she
has a pretty good chance of landing in the hospital with this stuff.
Linda L. Shaw MD FAAP