IMMUNITY. 1 5 



entered, and apparently multiplied to some extent in the 

 blood, whereas without such previous bleeding they were not 

 to be found in the circulating fluid. Certain anaesthetic 

 agents have also been shown to produce a similar result. 

 Platania communicated anthrax to immune animals — dogs, 

 frogs, pigeons — by bringing them under the influence of 

 curare, chloral, or alcohol ; and Wagner obtained similar re- 

 sults in his experiments upon pigeons to which he had admin- 

 istered chloral. In man, clinical experience shows that those 

 who are addicted to the excessive use of alcohol are especially 

 liable to contract certain infectious diseases, — pneumonia, 

 erysipelas, yellow fever, etc. 



The pathogenic potency of known disease germs varies as 

 widely as does the susceptibility of individuals to their specific 

 action. In general it may be said that the more recently the 

 germ comes from a developed case of the disease to which it 

 gives rise, the more virulent it is, and the longer it has been 

 cultivated outside of the animal body, the more attenuated is 

 its pathogenic power. Thus, when the discharges of a typhoid- 

 fever patient find their way directly to a water-supply of limited 

 amount, a large proportion of those who drink the water are 

 likely to be attacked ; but when a considerable interval of time 

 has elapsed since the contamination occurred, although the 

 germs may still be present, the liability to attack is much less, 

 on account of diminished pathogenic virulence. 



What has been said thus far indicates that infection depends 

 {a) upon the susceptibility of the individual (predisposition), 

 {b) upon the virulence of the infectious agent, and {c) upon 

 the quantity introduced to a vulnerable point, — e.g. beneath 

 the skin by inoculation or through an accidental wound in 

 anthrax and other forms of septicaemia, into the alimentary 

 canal in typhoid fever and cholera, or into the respiratory pas- 

 sages in diphtheria, influenza, pneumonia, and pulmonary tuber- 

 culosis. Still another factor may be called into play. In 

 addition to the specific cause or germ, and the natural or 

 acquired (by various depressing agencies referred to) predis- 

 position, a direct exciting cause may be necessary to establish 

 a localized infectious process. Thus, catching cold may be the 



