While a puppy is nursing, it may receive protective antibodies in its mother's milk. As soon as the puppy is weaned, this natural immunity will begin to disappear and may be gone within two weeks.
However, it is your job to keep your dog from getting sick. Let us assume you start with a puppy.
Distemper, hepatitis and leptospirosis are common and serious diseases which destroy many pets each year.
Your veterinarian may want to give your puppy immediate temporary protection at the time of purchase or adoption with a "puppy shot" of antiserum which contains antibodies against distemper, hepatitis and leptospirosis as well as some of the other common diseases.
Vaccinations provide long term immunity and most puppies will be started on a series of vaccinations on their first visit to the veterinarian.
Booster vaccines are then advisable on a regular basis for adult dogs, to maintain his overall health for years to come.