Did you know that when puppies are born, they have not yet developed their immune system? It takes 4 weeks for their bodies to develop this protection. I seldom let anyone handle my puppies until they are about a month old. They are susceptible to any disease or infection that a visitor may track in. Shots are not given until puppies are 6 weeks old. If your litter is healthy and thriving at four weeks old, they should have a good immune system in place.