Authorities are still divided as to the meaning and even the precise frequency of the occurrence of the pneumococcus in the healthy human mouth. Some hold that its presence is due to recent infection which has either been unable to gain entrance to the system or is preparing its attack; others, that it is a survival from some previous mild attack of the disease, and the body tissues having acquired immunity against it, it remains in them as a harmless parasite, as is now well known to be the case with the germs of several of our infectious diseases--for instance, typhoid--for months and even years afterward. Others hold the highly suggestive view that it is a normal inhabitant of the healthy mouth, which can become injurious to the body, or _pathogenic_, only under certain depressed or disturbed conditions of the latter. In defense of this last it may be pointed out that dental bacteriologists have now already isolated and described some thirty different forms of organisms which inhabit the mouth and teeth; and the pneumococcus may well be one of these. Further, that a number of our most dangerous disease germs, like the typhoid bacillus, the bacillus of tuberculosis, and the bacillus of diphtheria, have almost perfect "doubles," law-abiding relatives, so to speak, among the germs that normally inhabit our throats, our intestines, or our immediate surroundings. The ultimate foundation question of the science of bacteriology is, How did the disease germs become disease germs? But the question is still unanswered.