The term 'microbial persistence' describes a phenomenon whereby microorganisms which are drug-susceptible when tested outside the body are nevertheless capable of surviving within the body despite intensive therapy with the appropriate antimicrobial drug. In clinical practice this phenomenon obviously has to do with the post-treatment 'carrier state' and with post-treatment relapse. In short, it is this phenomenon which is responsible for our inability to eradicate an infection from a person or a community by the use of drugs. - Walsh McDermott, The Yale Journal of Biology and Medicine 30, 257 (1958).