Puppies and Parvo Vaccinations?

If someone can explain how a puppy leaves its litter healthy and fat and full of life and the new owners take the puppy to a vet a day or two after. The vet gives the puppy its shots and checks for worms and pravo and everything is okay, but then four or five days later the puppy starts to vomit and pooping diarrhea and then is diagnosed with pravo. I have heard this a lot, the puppies are all getting sick days after getting their shots, confused?

By Rafi