SYMPOSIUM ON PUBLIC HEALTH PROBLEMS 83 
Two. Control of Carriers. 
1. Lower animals as carriers. Some diseases may be 
brought under practically complete control by the control of 
the animal species which are their sole means of dissemination. 
Such are yellow fever, and malaria, disseminated by mosqui- 
toes, the stegomyia and amopheles, respectively ; typhus fever, 
which is louse borne, and bubonic plague which is carried from 
rodents, especially rats, to man, by fleas. To this list may be 
added Rocky Mountain spotted fever which is conveyed to 
man by a species of tick. 
2. Human Carriers. Aside from the agency of patients in 
spreading diseases, we are forced to recognize the fact that 
persons who have acquired a large degree of immunity may 
harbor, especial in body passages, disease germs which when 
transplanted to non-immunes, may have pathogenic results 
often of an epidemic character. There are such typhoid and 
diphtheria carriers. There are such epidemic cerebro-spinal 
meningitis and possibly acute poliomyelitis carriers. 
The venereal diseases, especially syphilis and gonorrhoea, 
are conveyed at times by those who are not suffering in an 
obviously acute manner from these diseases, though they can- 
not be strictly said to be immune carriers. 
The control of human carriers is obviously fraught with 
difficulties. But we hope that the growth of enlightenment 
will give society the power to identify and control such car- 
riers and that the increase of good will among men “when this 
cruel war is over,” will make all carriers ready to be cleared of 
any agency in the spread of disease. 
THREE. Control of Diseases by Biological Agents. 
There are reactions and reinforcements which may be pro- 
duced or introduced into the human system which are positive 
in their preventive effects. Vaccination in the case of small- 
pox; anti-typhoid and anti-paratyphoid inoculation in the 
case of these diseases; anti-tetanic serum in tetanus and diph- 
theria anti-toxin are examples of positive control of the re- 
spective diseases. To these may be added anti-plague, anti- 
