One of the biggest dangers of parvo is dehydration because of the
vomiting and diarrhea. If the puppy becomes dehydrated, the body's electrolytes are thrown off and all kinds of problems, not the least of which is a disruption of the heart rhythm, can occur.
If the puppy is at home, give him Pedialyte. If he will not drink it himself, force it in with an eyedropper or a syringe.