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.

There are also mixed infections in which two different organisms unite in attack, each in some way assisting in the action of the other.  The best known example of this is in the highly infectious disease of swine known as hog cholera.  It has been shown that in this disease two organisms are associated,—­one an invisible and filterable organism, and the other a bacillus.  It was first supposed that the bacillus was the specific organism; it was found in the lesions and certain, but not all, the features of the disease were produced by inoculating hogs with pure cultures.  The disease so produced is not contagious, and the contagious element seems to be due to the filterable virus.