PREVENTIVE MEDICINE AND THE WAR 345 



which had prevailed in the various army camps during the 

 previous year. Our justification for this statement is that 

 there are many incidents of people who largely escaped the 

 disease in the fall and who had experienced or lived through 

 a similar but much milder epidemic during the previous spring. 

 In other words, we may say the soldiers who had clinical in- 

 fluenza in camp prior to August and those who, although hav- 

 ing no clinical manifestations of the disease, lived through 

 such an epidemic, were less gravely affected when the more 

 virulent organisms reached them in the fall. 



4. We believe that the largest single factor influencing the 

 spread of influenza is the susceptibility of the individuals among 

 whom it has been introduced. If these individuals have been 

 once attacked by the disease even in a mild form or lived 

 through a mild epidemic without showing clinical symptoms, 

 they suffer less when the disease is again introduced. Among 

 communities not previously exposed to influenza, this dis- 

 ease has usually involved from 20 to 50 per cent, of the per- 

 sonnel. The exact number affected is determined by the num- 

 ber of people who are naturally immune or have secured im- 

 munity by previous exposure. 



5. It appears that natural immunity gives way before ex- 

 posure, over-work and fatigue, as was demonstrated years ago 

 by Pasteur in his experiments on birds with anthrax. Like- 

 wise, it is possible for human beings to have their resistance 

 lowered by exposure to unaccustomed environment so that al- 

 though naturally immune, the standard of immunity is re- 

 duced to the point where the influenza virus gains admittance, 

 and overcomes the lowered resistance. 



6. We believe that not only the rapidity with which the dis- 

 ease spreads, but its virulence is in direct proportion to the 

 density of the susceptible population. In communities such 

 as army camps and large cities, the contact of individuals is 

 so close and so intimate that even though extra precautions 

 are taken it is quite impossible to prevent disease from ul- 

 timately reaching all persons. Precautions such as quaran- 



