The development of purified vaccines for the control of common infectious diseases may lead to a sense of false security and a tendency to over-immunize. Repeated antigenic challenge may cause hypersensitivity reactions which could harm the individual. For each vaccine there is an opitmum time and age for dosing; for those vaccines requiring a course of doses, there is a minimum but no maximum time interval between the doses. In Australia, active immunization is conducted on a regular basis against seven preventable infectious diseases. In this article, the vaccine control of each disease is discussed briefly and suggestions made to avoid hyperimmunization. The need to appreciate the dangers inherent in all immunization procedures in general, and in certain vaccines in particular, is emphasized. Indescriminate immunization cannot be condoned.