There is a bacteria called campylobacter that mimics parvo even the blood test shows a positive for parvo. If the dog is fully vaccinated then this should be something your vet should consider.
Here'a an article on Parvo vs. Campylobacter
http://nypugclub.org/health/ShowCrud.htm
Like others have said, parvo symptoms are very severe, the dog will not eat (even the most tempting foods) or drink and usually hide and of course have the smelly diarrhea and vomiting.
It is true that the virus can remain active for a year or more. I would take a solution of bleach and water and spray your entire yard.