PARASITIC BACTERIA. 15 



the first place, that commonly after such a recov- 

 ery the individual has decidedly increased resist- 

 ance to the disease. This increased resistance 

 may be very lasting, and may be so considerable 

 as to give almost complete immunity from the 

 disease for many years, or for life. One attack 

 of scarlet fever gives the individual great immu- 

 nity for the future. On the other hand, the re- 

 sistance thus derived may be very temporary, as 

 in the case of diphtheria. But a certain amount 

 of resistance appears to be always acquired. 

 This power of resisting the activities of the para- 

 sites seems to be increased during the progress 

 of the disease, and, if it becomes sufficient, it 

 finally drives off the bacteria before they have 

 produced death. After this, recovery takes place. 

 To what this newly acquired resisting power is 

 due is by no means clear to bacteriologists, al- 

 though certain factors are already known. It 

 appears beyond question that in the case of cer- 

 tain diseases the cells of the body after a time 

 produce substances which serve as antidotes to 

 the poisons produced by the bacteria during their 

 growth in the body antitoxines. In the case of 

 diphtheria, for instance, the germs growing in the 

 throat produce poisons which are absorbed by the 

 body and give rise 'to the symptoms of the dis- 

 ease; but after a time the body cells react, and 

 themselves produce a counter toxic body which 

 neutralizes the poisonous effect of the diphtheria 

 poison. This substance has been isolated from 

 the blood of animals that have recovered from an 

 attack of diphtheria, and has been called diphthe- 

 ria antitoxine. But even with this knowledge the 

 recovery is not fully explained. This antitoxine 

 neutralizes the effects of the diphtheria toxine, 

 11 



