ABSTRACT
ConcurrentwiththediscoveryofthecauseofinfectiousabortionincattlebyBang,it wasknownthatinfectedanimalsoftendevelopedresistancetosubsequentclinicalsigns. Manyworkerslaterconductedexperimentsusingbothkilledandlivecultures.Thekilled onesconferredpoorimmunity.Thelivevaccineswereusuallyofvirulentstrainsadministered beforethecattlebecamepregnant.Someprovidedgoodimmunityagainstabortionbut frequentlyledtoexcretionofthepathogenicorganismsandinfectionofsusceptiblecattle.
