26 INFECTION AND IMMUNITY 



influence of insufficient food is most marked, and in 

 times of distress, due to failure of the food supply, any 

 infectious disease is liable to exhibit a malignancy 

 and fatal character, although under ordinary conditions 

 it may be comparatively harmless. Insanitary sur- 

 roundings, by vitiating the air, insufficient ventilation 

 and overcrowding of dwellings, factories, school- 

 houses, etc. all have a tendency to lower the vital 

 resisting power of individuals subjected to such in- 

 fluences and to increase the susceptibility to infec- 

 tion. Debility resulting from loss of blood or the 

 exhaustion following great fatigue also increases the 

 susceptibility to various infectious diseases. Clinical 

 observation shows that a similar result follows the ex- 

 cessive use of alcoholic drinks. 



Localised infectious processes are not only more 

 liable to be established in individuals whose vital 

 energy is reduced by any of the causes mentioned, 

 but also as a result of causes which reduce the resist- 

 ing power of the tissues at the point of invasion. 

 Thus a carbuncle or an abscess may develop in tis- 

 sues that have been bruised or injured in any way ; 

 and the congestion or inflammation of the fauces 

 which is so common as a chronic or acute condition 

 a "sore throat"- no doubt increases the suscepti- 

 bility to diphtheritic infection in this locality. It is 

 well known that pneumonia often follows attacks of 



