from internet:
Window of susceptibility The age at which young animals can effectively be immunized is proportional to the amount of antibody protection they received from their mother. In the first weeks of a puppy's life, high levels of maternal antibodies present in the puppy's bloodstream block the effectiveness of most vaccines. With time, the amount of maternal antibodies in the puppy drops to a low enough level in the puppy that immunity (protection from disease) can be produced through vaccination. This usually occurs between five and twelve weeks of age. There is a period of time from several days to several weeks in which the maternal antibodies are too low to provide protection against disease, but too high to allow a vaccine to work. This period is called the 'window of susceptibility'. This is a time period when, despite being vaccinated, a young animal can still contract a disease.
