Were all maladies whose causation is evidently of external origin capable of the same clear discrimination as these, we should have no difficulty with the present topic. But, in fact, no subject connected with {136} the history of disease has become surrounded by more intricate controversy. Many times the same facts are, or appear to be, explicable in two or three different ways. What some hold to be proofs of contagion from person to person, others are ready to account for by the subjection of a number of persons or of a whole community to either a common local or a widespread migrating (epidemic) influence. It is sometimes impossible, in the nature of things, to obtain an absolute demonstration of the truth of one or another of these theories without such experiments upon human beings as are impracticable. While endeavoring to ascertain the limits of our present knowledge upon these questions, let us first notice what are the most positive facts concerning them, some of which are common to the whole group or class of what have been, since Liebig, often called zymotic,[7] but latterly more often enthetic, diseases. [Footnote 7: The term zymotic has, with many authors, fallen into disrepute, chiefly because Liebig's hypothesis concerning the chemico-physical action of ferments, as well as of contagia, has lost ground in comparison with the vital or disease-germ theory. Yet the analogy between fermentation, putrefaction, and the action of a virus on an animal organism persists; whatever may be the theory of their explanation, something appears to be common or similar in all these processes.] These diseases may be enumerated as follows: 1. _Only produced by contact or inoculation_. Primary Syphilis, Gonorrhoea, Vaccinia, Hydrophobia. 2. _Contagious also by atmospheric transmission through short distances_. Variola, Varioloid, Varicella, Measles, Diphtheria, Scarlatina, Rötheln, Mumps, Whooping Cough, Typhus, Relapsing Fever. 3. _Endemic, occasionally epidemic_. Malarial Fevers (Intermittent, Remittent, and Pernicious Fever), Dengue, Yellow Fever. 4. _Other zymotic or enthetic diseases_. Influenza, Cerebro-spinal Fever, Erysipelas, Puerperal Fever, Tropical Dysentery, Typhoid Fever, Cholera, Plague.