Here is a really good article on Parvo:
http://www.peteducation.com/article....&articleid=467
You will notice it says that the main cause of parvo is vaccinations interfering with the mother's immunities. Any immunity a puppy may still have from nursing is often killed off by the first vaccinations leaving a puppy completely unprotected. People mistakenly think since a puppy has had one or even two rounds of shots, he will be safe.
Puppies should not be allowed in areas where other dogs have been until two weeks after they get their final round of shots. That's usually about 4&1/2 months. Until then, keep them out of grooming salons, pet stores, dog parks, etc. Make sure you have him in your arms or a carrier when you visit the vet. Don't walk him in areas other dogs are walked like common areas in apartment complexes or neighborhoods. Train him to use pads instead, then transition him to outside if that's what you prefer.
Parvo is so deadly. We had a puppy on another forum get parvo from his own backyard. It wasn't fenced in and loose dogs used it for a bathroom.