As I understand it, the puppies will have tha antibodies from the mother
for viral infections that she has been exposed to. If a dog comes onto the premises carrying a virus to which the bitch has not been exposed, then the puppies cannot have antibodies which would protect them from such a virus.
For this reason, when we've had pups, I've always insisted that anyone coming in sloshes through a mat which is soaked in parvoviricide, and I used to ask them to wash their hands before touching any puppies (not that anyone is allowed to touch puppies under 2 weeks apart from family, who would handle them all the time - after washing hands of course.
I would not let any other dogs "visit" during this time either. And, I would also ask that no-one comes to my home after having visited othr puppies on the same day, nor go to visit other puppies the same day.
Since I had to fight to save a litter of kittens after they had been exposed to cat flu - after someone came to view my kittens and said "oh these are so much healthier than the ones we saw this morning



- I make it a point of saying to anyone coming to view "please do not look at any other puppies the day you come to see mine - you could bring/take cross infection".
Margot