to combine with both the bacterial cell and the complement which brings about its destruction, and just as antitoxine is formed so new amboceptors may be formed. Few hypotheses have been advanced in science which are more ingenious, in better accord with the facts, have had greater importance in enabling the student to grasp the intricacies of an obscure problem, and which have had an equal influence in stimulating research. The immunity which results from disease in accordance with this theory, is due not to conditions preventing the entrance of organisms into the body, but to greater aptitude on the part of the cells to produce these protective substances having once learned to do so. An individual need not practise for many years, having once learned them, those combinations of muscular action used in swimming; but the habit at once returns when he falls into the water. Infectious diseases and recovery are phases of the struggle for existence between parasite and host, and illustrate the power of adaptation to environment which is so striking a characteristic of living matter. FOOTNOTES: [1] The comparison here is with the atrium of a Pompeiian house. CHAPTER VIII SECONDARY, TERMINAL AND MIXED INFECTIONS.--THE EXTENSION OF INFECTION IN THE INDIVIDUAL.--TUBERCULOSIS.--THE TUBERCLE BACILLUS.--FREQUENCY OF THE DISEASE.--THE PRIMARY FOCI.--THE EXTENSION OF BACILLI.--THE DISCHARGE OF BACILLI FROM THE BODY.--INFLUENCE OF THE SEAT OF DISEASE ON THE DISCHARGE OF BACILLI.--THE INTESTINAL DISEASES.--MODES OF INFECTION.--INFECTION BY SPUTUM SPRAY.--INFECTION OF WATER SUPPLIES.--EXTENSION OF INFECTION BY INSECTS.--TRYPANOSOME DISEASES.--SLEEPING SICKNESS.--MALARIA.--THE PART PLAYED BY MOSQUITOES.--PARASITISM IN THE MOSQUITO.--INFECTION AS INFLUENCED BY HABITS AND CUSTOMS.--HOOKWORM DISEASE.--INTER-RELATION BETWEEN HUMAN AND ANIMAL DISEASES.--PLAGUE.--PART PLAYED BY RATS IN TRANSMISSION.--THE PRESENT EPIDEMIC OF PLAGUE. The infectious diseases are often complicated by secondary infections, some other organism finding opportunity for invasion in the presence of the injuries produced in the primary disease. In many diseases, such as diphtheria, scarlet fever and smallpox, death is frequently due to the secondary infection. The secondary invaders not only find local conditions favoring a successful attack, but the activity of the tissue cells on which the production of protective substances essentially depends has suffered by the primary infection, or the cells are occupied in meeting the exigencies of this. The body is in the position of a state invaded by a second power where all its forces and resources are engaged in repelling the first attack. What are known as terminal infections occur shortly before death. No matter what the disease which causes death, in the last hours of life the body usually becomes invaded by organisms which find their opportunity in the then defenceless tissues, and the end is often hastened by this invasion.