232 SUSCEPTIBILITY AND IMMUNITY. 



In the infectious diseases of man involving the system generally, 

 a single attack commonly confers immunity from subsequent attacks. 

 This is true of the eruptive fevers, of typhoid fever, of yellow fever, 

 of mumps, of whooping cough, and, to some extent at least, of syphi- 

 lis. But it seems not to be the case in epidemic influenza (la grippe), 

 in croupous pneumonia, or in Asiatic cholera, in which diseases 

 second attacks not infrequently occur. In localized infectious dis- 

 eases such as diphtheria, erysipelas, and gonorrhoea one attack is not 

 protective. Croupous pneumonia and Asiatic cholera should per- 

 haps be grouped with diphtheria and erysipelas as local infections 

 with constitutional symptoms resulting from the absorption of toxic 

 products. But typhoid fever, mum^s, and whooping cough, in 

 which one attack gives immunity, are also localized infectious dis- 

 eases. 



We are therefore not able to group infectious diseases into two 

 classes, in one of which there is a general infection followed by im- 

 munity, and in the other a local infection without subsequent immu- 

 nity. Indeed, in the eruptive fevers and specific febrile infectious 

 diseases generally the immunity following an attack is not abso- 

 lute. Second attacks of small-pox, of scarlet fever, and of yellow 

 fever occur occasionally, although a large majority of those who suf- 

 fer an attack of one of these diseases have an immunity for life. On 

 the other hand, in the diseases mentioned in which one attack is not 

 generally recognized as protecting from future attacks, it is probable 

 that a certain degree of immunity, of limited duration perhaps, is 

 acquired. In localized infection, as in gonorrhoea or erysipelas, the 

 invaded tissues appear after a time to acquire a certain tolerance to 

 the pathogenic action of the invading parasite, and no doubt recovery 

 from these diseases would in many cases occur, after a time, without 

 medical interference. In diphtheria, cholera, and epidemic influenza 

 second attacks do not often occur during the same epidemic, and 

 there is reason to believe that a recent attack affords a certain degree 

 of immunity. 



That immunity may result from a comparatively mild attack as 

 well as from a severe one is a matter of common observation in the 

 case of small-pox, scarlet fever, yellow fever, etc. ; and since the dis- 

 covery of Jenner we have in vaccination a simple method of produc- 

 ing immunity in the first-mentioned disease. The acquired immunity 

 resulting from vaccination is not, however, as complete or as per- 

 manent as that which results from an attack of the disease. 



These general facts relating to acquired immunity from infectious 

 diseases constituted the principal portion of our knowledge with re- 

 ference to this important matter up to the time that Pasteur (1880) 

 demonstrated that in the disease of fowls known as chicken cholera, 



