182 IMMUNITY 



Then, of those who do develop the disease, some have it lightly, others 

 more severely and some fatally. The relative insusceptibility of nurs- 

 ing infants to diphtheria and certain other infections is well known, 

 and Schick has shown that the blood of many of them contains anti- 

 toxin, possibly acquired through the mother's milk. Certainly, no 

 such marked variations in susceptibility to a chemical poison could be 

 expected. An infection must meet with some resistance to its mul- 

 tiplication and this is not equally effective in all individuals. It seems 

 fair to conclude from this that in all probability the resisting agent is 

 also a living thing or some product or products of living things. 



