Nails can break if they are too long. So it may not be related to the parvo. If the nails are short and still breaking, you may want to check with your vet. If the nails are long, they need to cut or grinded - but only a little at a time as the longer the nail, the longer the quick (the part that bleeds). Stypic powder or stypic pads will stop the bleeding.
Sting Chance von Gaard AKC GSD 2/8/2006