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.