In concluding this subject I may observe, that one of the greatest hindrances to the due recognition of the remarkable part played by parasites in the production of human endemics and animal epizoötics arises from the circumstance that no inconsiderable number of minute worms may infest a host without obvious injury. This immunity proves nothing. If, for example, we take the case of _Trichina_ we find that several millions of entozoa may exist in the human, or, at all events, in the animal bearer, without producing any symptom of discomfort. In such cases it is not possible to determine the strict limits of health and disease; nevertheless, were we to double the amount of infection, the imaginary line of demarcation is at once bridged over and the parasites become acknowledged as directly responsible for grave symptoms which may even prove fatal to the bearer. Again, the relative strength and size of the infected host constitute factors that materially limit the power of the parasite for injury. Where the entozoa are of minute size, and where their injurious action is primarily due to the mechanical obstructions they set up, it is clear that the virulence of the helminthiases, or resulting diseased conditions, will mainly depend upon the number of intruders. Another consideration of the highest value in relation to epidemiology generally, and more especially in regard to the practical question as to the best methods of stamping out parasitic plagues, is that which refers to the life-history of the entozoon itself. It must be obvious that in all cases where the intermediate host can be captured and destroyed, the life-cycle of the parasite can be broken and interrupted, and if thus broken, there is an end to the further propagation of the species. The knowledge that we have acquired by experimental research in this connection has already enabled us to set a limit upon the prevalence of certain well-known disorders, such as Trichinosis, Cestode-tuberculosis, and so forth. In the case of epizoötics, however, which are indirectly due to the action of intermediary hosts that cannot be readily captured or destroyed, then our power of arresting the disease is comparatively limited. In the present case it is probably not necessary either that a dead or living mosquito should be swallowed to insure infection; but it _is_ necessary that the parasitic larvæ should have dwelt within the mosquito in order to arrive at the highest stage of larval growth prior to their re-entrance within the human territory. Undoubtedly, the larvæ are swallowed with potable waters. Perfect filtration before use would certainly check, if in course of time it did not totally extinguish several of the many virulent diseases that now afflict the inhabitants of warm climates. It is with reluctance that I terminate this article, but in the closing pages of this work (Book II, Section V) I hope to add a few more particulars in reference to Lewis’s latest researches.