Parvo is a highly infectious viral disease in dogs (primarily puppies).
When I was working at the veterinary hospital, we saw a lot of Pet Store puppies that came in with this disease. It was such a sad thing to see. You could look in their eyes, and there was just no life or happiness there. The symptoms of the puppies that came in with this disease included: diarrhea, vomiting, gagging, high temperature (fever) and dehydration.
It was rewarding to see a puppy recover from this disease after weeks of treatment, but you are right in that it's a difficult process, and expensive to treat.
Unfortunately many of the puppies also died from it.

Here's some more info: http://www.vetinfo4dogs.com/dparvo.html