If this view of the original character and taming of these diseases be correct, it also accounts for the extraordinary and otherwise inexplicable cases where they suddenly assume the virulence of cholera, or yellow fever, and kill within forty-eight or ninety-six hours, not merely in children but also in adults. To group these three diseases together simply because they all happen to occur in children would appear scarcely a rational principle of classification. Yet, practically, widely different as they are in their ultimate results and, probably, in their origin, they have so many points in common as to their method of spread, prevention, and general treatment, that what is said of one will with certain modifications apply to all. I said "probably" of widely different origin, because, by one of those strange paradoxes which so often confront us in real life, though the infectiousness and the method of spread of all these diseases is as familiar as the alphabet and as firmly settled, the most careful study and innumerable researches have failed to identify positively the germ in any one of them. There are a number of "suspects" against which a great deal of circumstantial evidence exists: a streptococcus in scarlet fever, a bacillus in whooping-cough, and a protozoan in measles; but none of these have been definitely convicted. The principal reason for our failure is a very common one in bacteriological research, whose importance is not generally known, and that is, that there is not a single species of the lower animals that is subject to the diseases or can be inoculated with them. This unfortunate condition is the greatest barrier which can now exist to our discovery of the causation of any disease. We were absolutely blocked, for instance, by it in smallpox and syphilis until we discovered that our nearest blood relatives, the ape and the monkey, are susceptible to them; and then the _Cytoryctes Variolae_ and the _Treponema pallida_ were discovered within comparatively a few months. Some lucky day, perhaps, we may stumble on the animal or bird which will take measles, scarlet fever, or whooping-cough, and then we will soon find out all about them.