Piroplasmoses in South Africa among cattle and horses, and in other countries are transmitted in similar ways. Probably many of the diseases due to spirochetes and trypanosomes are likewise transmitted by _necessary_ insect intermediaries. In human medicine the eradication of yellow fever from Panama and Cuba is due to successful warfare against, a certain mosquito (_Stegomyia_). So the freeing of large areas in different parts of the world from _malaria_ follows the destruction of the mosquitoes. The prevention of typhus fever and of trench fever by “delousing” methods is familiar from recent army experience though for typhus this method has been practiced in Russia for more than ten years to the author’s personal knowledge. The campaign against disease in animals and man from insect sources must be considered as still in its infancy. The full utilization of tropical lands depends largely on the solution of this problem. CHAPTER XXIV. PATHS OF ENTRANCE OF PATHOGENIC ORGANISMS, OR CHANNELS OF INFECTION. _A._ =The Skin.=--If the skin is healthy there is no opportunity for bacteria to penetrate it. It is protected not only by the stratified epithelium, but also in various animals, by coats of hair, wool, feathers, etc. The secretion pressure of the healthy sweat and oil glands acts as an effective bar even to motile bacteria. Nevertheless a very slight injury only is sufficient to give normal surface parasites and other pathogenics, accidentally or purposely brought in contact with it, an opportunity for more rapid growth and even entrance for general infection. Certain diseases due to higher fungi are characteristically “skin diseases” and rarely become general--various forms of favus, trichophyton infections, etc. A few disease organisms, tetanus, malignant edema, usually get in through the skin; others, black-leg, anthrax, quite commonly; and those diseases transmitted by biting and blood-sucking insects, piroplasmoses, trypanosomiases, spirilloses, scarcely in any other way. Defective secretion in the skin glands from other causes, may permit lodgment and growth of bacteria in them or in the hair follicles. “Pimples” and boils in man and local abscesses occasionally in animals are illustrations. Sharp-edged and freely bleeding wounds are less liable to be infected than contusions, ragged wounds, burns, etc. The flowing blood washes out the wound and the clotting seals it, while there is less material to be repaired by the leukocytes and they are free to care for invading organisms (phagocytosis). Pathogenic organisms, especially pus cocci, frequently gain lodgment in the _milk glands_ and cause local (mastitis) or general infection. _B._ =Mucosæ directly continuous with the skin and lined with stratified epithelium= are commonly well protected thereby and by the secretions.