Very young puppies and kittens are highly susceptible to infectious diseases because their immune systems are not fully mature. While nursing, their mother’s milk contains antibodies (special proteins) that provide some immunity to diseases; however, these maternal antibodies do not last long, and there may be gaps in protection as the milk antibodies decrease and the puppies’ or kittens’ immune system isn’t yet capable of fighting off infection. In many instances, the first dose of a vaccine serves to prime the pet’s immune system against the virus or bacteria while subsequent doses help to further stimulate the immune system to produce the antibodies needed to protect a pet from specific diseases. To keep these gaps in protection as small as possible and to provide optimal protection against disease in the first few months of life, a series of vaccinations are scheduled, usually 3-4 weeks apart. For most puppies and kittens, the final vaccination in the series is administered at about 4 months of age; however, in some situations, a veterinarian may alter this schedule based on an individual animal’s risk factors. Remember that an incomplete series of vaccinations may lead to incomplete protection, making puppies and kittens vulnerable to infection.