alive by transmission from animal to animal, and
the persistence of the infection is favored by mild and chronic cases.

CHAPTER IX

DISEASE CARRIERS.--THE RELATION BETWEEN SPORADIC CASES OF INFECTIOUS
DISEASE AND EPIDEMICS.--SMALLPOX.--CEREBRO-SPINAL
MENINGITIS.--POLIOMYELITIS.--VARIATION IN THE SUSCEPTIBILITY OF
INDIVIDUALS.--CONDITIONS WHICH MAY INFLUENCE SUSCEPTIBILITY.--RACIAL
SUSCEPTIBILITY.--INFLUENCE OF AGE AND SEX.--OCCUPATION AND
ENVIRONMENT.--THE AGE PERIOD OF INFECTIOUS DISEASES.

We have seen that insects serve as carriers of disease in two ways: in
one, by becoming contaminated with organisms they serve as passive
carriers, and in the other they undergo infection and form a link in
the disease. The more recent investigations of modes of transmission
of infectious diseases have shown that man, in addition to serving
while sick as a source of infection, may serve as a passive carrier in
two ways. For infection to take place not only must the pathogenic
organism be present, but it must be able to overcome the passive and
active defences of the body and produce injury. Pathogenic organisms
may find conditions favorable for growth on the surfaces of the body,
and may live there, but be unable to produce infection, and the
individual who simply harbors the organisms can transmit them to
others. Such an individual may be a greater source of infection than
one with the disease, because there is no suspicion of danger. The
organisms which thus grow on the surfaces have in some cases been
shown to be of diminished virulence, but in others have full
pathogenic power. Such passive carriers of infection have been found
for a number of diseases, as cerebro-spinal meningitis, diphtheria,
poliomyelitis and cholera. In all these cases the organisms are most
frequently found in those individuals who have been exposed to
infection as members of a family in which there have been cases of
disease. The other sort of carrier has had and overcome the disease,
but mutual relations have been established with the organism which
continues to live in the body cavity. Diphtheria bacilli usually
linger in the throat after convalescence is established, and until
they have disappeared the individual is more dangerous than one
actually sick with the disease. Health officers have recognized this
in continuing the quarantine against the disease until the organism
disappears. In typhoid fever bacilli may remain in the body for a lo