504 INFECTION AND RESISTANCE 



existence of a true immunity analogous to that following typhoid, 

 plague, etc. For we have seen that a very definite insusceptibility 

 is coincident with the persistence of actual disease in the subject 

 and, as Neisser points out, the more recent investigations carried out 

 with the aid of serological tests have shown that the number of cases 

 uncured though long without symptoms, is much larger than for- 

 merly supposed. The scarcity of true reinfection, therefore, may 

 well be due to the relative scarcity of completely cured cases. More- 

 over, it must be remembered that, in this disease, even when final 

 recovery results, it is usually achieved only at an age when the in- 

 dividual is less exposed to reinfection because of changed economic 

 and domestic conditions, or by reason of the virtue which comes with 

 arteriosclerosis and the wisdom of the burnt child that fears the fire. 



Granted, then, that true reinfection is possible, is there any evi- 

 dence that when it does occur, the second attack is less severe and 

 more easily cured than the first, a fact which would also tend to 

 support the opinion that a certain degree of immunity persists. 

 Jonathan Hutchinson 134 expressed this view in a clinical lecture in 

 which he states that "second chancres are far more common than 

 second attacks of constitutional syphilis." However, John in his 

 summary, in which Hutchinson's cases are included, finds that in 

 general the disease has run a second course very similar as to severity 

 to that incident to the first infection. 



If we gather together, then, the facts revealed by clinical study we 

 may conclude with Levaditi, Neisser, and most others, that : 



1. The syphilitic subject acquires definite resistance to rein- 

 oculation which becomes manifest soon after the appearance of the 

 primary sore, at a time when the virus may be regarded as having 

 gained universal systemic distribution. 



2. This resistance, high though not absolute, persists through- 

 out the secondary or most active period of the disease and into the 

 tertiary stage. During the latter, however, it appears somewhat to 

 decline, reinoculation or superinfection being more frequently pos- 

 sible at this period. 



3. AVhen syphilis is entirely cured, susceptibility may in all 

 probability be regarded as returning, possibly, though not certainly, 

 to the same degree as it exists in the normal subject. The reasons 

 for this last belief will become more clear when we study the evidence 

 contributed by animal experimentation. 



Notwithstanding the admirable thoroughness with which clinical 

 data had been collected and analyzed in the study of syphilis, prog- 

 ress beyond the points indicated in the preceding sections was quite 

 impossible without the aid of animal experimentation and a knowl- 

 edge of the causative agent. Fortunately these two deficiencies in 

 our methods were removed when in 1903 Metchnikoff and Roux 

 134 Hutchinson, J. Brit. Med. Jour., 1882, i, 699. 



