The virus likes to grow in rapidly dividing cells. The intestinal lining has the biggest concentration of rapidly dividing cells in a puppy's body. This is a very serious disease. Some puppies infected with parvovirus will die despite prompt and adequate treatment. Due to the high death rate, parvovirus gets a lot of free publicity.
Many people just assume that any case of diarrhea in a dog is from parvovirus.
This is not true. There are a lot of other diseases and disorders that lead to diarrhea. If you have a puppy, don't take any chances. Have your puppy examined by your vet if diarrhea is a factor in any disease. It is better to be safe than to be sorry.