The parvo virus can live on surfaces for months. Unless your yard is fenced in and no other dogs can get into it, it is not safe to let your puppy outside. It is very important that your puppy did not get it's first shots before nine weeks old and gets a final set of shots between 16-18 weeks, too.
We had a little Maltese on another forum get parvo from his own backyard when he was eight months old. His owners thought Tucker was fully protected, but his vaccinations were given too soon and were blocked by maternal antibodies from nursing.
Here is a great article for you:
Parvovirus: Serious Diarrhea in Puppies & Dogs