The theory proposed is that a critical level of specific serum IgG is sufficient to confer protection against infectious diseases by inactivating the inoculum of the pathogen. This theory relies heavily on evaluation of licensed vaccines and includes the following: Measurement of serum antibodies only reliably predicts the efficacy of vaccines, according to regulatory agencies. Serum IgG antibodies alone account for the protection conferred by passive immunization. "Herd" immunity conferred by vaccines on viral and bacterial diseases is best explained by serum antibodies that inactivate the inoculum on mucosal surfaces, thus reducing the pathogen's transmission. Once the disease is manifest, serum antibodies induced by active immunization will neither relieve symptoms nor eliminate the pathogen; specific IgG must be present when the host encounters the pathogen in order to confer protective immunity. Information about the initial pathogen-host contact is vital, whereas knowledge of the symptomatology of the disease may not be essential for vaccine development.