In basic veterinary education, a key concept in preventing infectious disease is to isolate the healthy animal away from sick animals. However, few veterinary schools teach the value of socialization. As a result, for decades veterinarians have been telling puppy owners to isolate their puppy from other dogs until their vaccinations are finished at 4 months of age. 

This is now outdated advice because the lack of social skills is a major reason for behavior problems, and eventual surrender and euthanasia.

The real risk of death from behavior surrender, is much greater than the risk of death due to infectious disease. The current recommendation should be to give the puppy a series of vaccinations AND socialization experiences both starting at 2 months of age, and continuing throughout life.

Veterinarians who look at the newest data are realizing that healthy puppies meeting friendly healthy dogs is no health risk, and is in fact, one of the best investments in developing a canine companion that is friendly with both people and other dogs.

As a practicing veterinarian, my current recommendation for new puppy owners, is to try to have their puppy meet 20 new healthy, friendly dogs (all ages) before the puppy is 6 months old.  

My clinical impression is that dogs that are socialized develop a stronger immune system, and are less likely to succumb to infectious disease.

Therefore, during puppy socialization, AVOID two classes of dogs:  1) those who appear unhealthy, or 2) those lacking in social skills. 

Dogs lacking in social skills appear fearful, aggressive, or over-exuberant. This is a dog who as a puppy was likely owned by someone either ignorant of the importance of socialization, or too lazy to carry it out.