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.

The modes of transmission of infectious diseases are of great
importance and are the foundation of measures of public health. In the
preceding chapter we have seen that in the infected individual the
disease extends from one part of the body to another. There is a
primary focus of disease fro