That sounds very strange to me. You see, I also bought puppy who started insane diarrhoea within two or three days. I took the puppy straight to the vet and they diagnosed giardia right away, from the sample of the diarrhoea. I can´t believe it took your veterinarian more than a month to come to the same conclusion!
However, thanks God they finally figured it out.
Anyways, if it is only giardia, don´t worry, it will be fine. My pup was at that time 7 weeks old, she got antibiotics and was on strict diet for 14 days (rice+boiled chicken) and after that everything was fine. She never had problems again. And we were not told any special precautions, nothing. Nobody catched it from the dog (it is hardly possible, if you simply wash your hands enough).
Good luck!