Even though dogs get vaccinations for parvo, it doesn't necessarily mean that they won't get it. I've seen too many at work that have been vaccinated for it and they still end up with it. There are many strains of the virus, some of them stronger than others because just like everything else that has a genetic code, it mutates and changes how weak or strong it is.
The very old and the very young are highly susceptible to the virus as well as certain breeds have a higher incidence of being prone to it. Parvo is spread through fecal matter, so with the habit that dogs have of smelling poop, they can pick it up. That's why we as their guardians need to be aware of what they are smelling. If you've ever smelled the feces of a dog that has parvo, you will NEVER forget the smell. It's hard to describe, but the smell is ingrained into your brain and you can't forget it.