The persistence in the body of pathogenic bacteria to which their host has grown immune is instanced by diphtheria bacilli in the throat and typhoid bacilli in the gallbladder of carriers. That the urethral crypts may harbor gonococci for many years is well known. In the solid tissues, as well as in the cavities and spaces of the body, bacteria may linger indefinitely without symptoms. I have seenBacillus pyocyaneus, a secondary invader, apparently disappear from a wound that continued to heal, only to reappear when the same tissues were opened again several months later.1I have also recorded a case of actinomycosis of the groin that appeared four times in twenty years, with six and seven year intervals of complete and apparently definitive healing.1The chronic bone abscess is an exquisite example of this longevity of organisms in the tissues, of the attenuation of bacterial virulence and of