PARVO

as halfbreed and mike said. make sure it aint coccidiosis. the symptoms are similar. the vet should have tested strickly for parvo to confirm or dispute any assumptions. if it were me, i would find a new that dont go off assumptions and guessing work, he got a MD in vet practices so he should have tested.

now you ask about getting rid of the virus. there are numerous chemicals that will kill the virus, from bleach to the expensive stuff the vets use to disinfect the offices. you can do as said before and bleach any and everywhere the dogs poo and pee, but the virus can not survive sunlight exposure, so if you have a pto driven tiller and a tractor, till your entire yard up, (SCREW THE GRASS IF YOU WANT TO GET RID OF THE VIRUS) take an electric sprayer and spray everywhere you tilled, give a day or so of sun light and then till it again and spray it again.
the virus can be transmitted by fly, flea, misquito, birds, people traffic, and so forth from a near by infected area, it can also be brought in by heavy rains and flooding carrying contaminated debris onto the land. no land is safe and i was like you, never inauculated my dogs till i had 3 come down with it from a dog that was a carrier but didnt die. i saved 2 of the 3 and now i vaccinate regularly just as a safety meassure.