Active immunization, achieved through proper vaccination, plays a critical role in the control of infectious diseases, both for individual cats and for the cat population as a whole. Some vaccines also reduce the potential for spread of zoonotic infections to humans (e.g., rabies). The benefits of routine, widespread vaccination are clear: the incidence of serious disease caused by pathogenic organisms, such as feline panleukopenia virus (FPV), can be reduced dramatically when widespread vaccination is practiced. However, the quality of vaccine-induced immunity is influenced by the patient’s environment, the characteristics of the vaccine, the pathogen, and the patient’s immune competence. Accurate prediction of the outcome of vaccination or the likelihood of exposure to a pathogen is impossible. Therefore, it is important that veterinarians inform cat owners that vaccination is not a guarantee of protection.