Parvovirus is a viral disease of dogs. It affects puppies much more frequently than it affects adult dogs. 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. The virus attacks and kills these cells, causing diarrhea (often bloody), depression and suppression of white blood cells -- which come from another group of rapidly dividing cells. In very young puppies it can infect the heart muscle and lead to "sudden" death. This is a very serious disease. Some puppies infected with parvovirus will die despite prompt and adequate treatment. While no extremely accurate statistics are available, a good guess is probably that 80% of puppies treated for parvovirus will live. Without treatment, probably 80% or more of the infected puppies would die.
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.