The test for Parvo can indeed cause a false positive if the dog was recently vaccinated with a live vaccine. Since the test is done with fecal matter, I can't imagine the vet not also running a standard fecal test to rule out roundworms, hookworms, whipworms, or coccidia at the same time. From what I understand, if a dog does recover from Parvo they can spread the disease in their stools for up to 6 months.

Here's some links to give you a bit more info if you're interested.



http://www.avma.org/animal_health/br...o_brochure.asp

Glad to hear your new foster is feeling better and hopefully the personality will gradually become more open and trusting in time.