FORTIFYING AGAINST DISEASE. 31 



causes do not always bring about the results which are observed 

 only in a certain percentage of cases. 



Effects of Individuality, Age, Sex, Family, Race, Spe- 

 cies. It is a matter of common experience that in times of epi- 

 demics persons equally exposed to infection are not all affected. 

 The weak members of the community are generally more readily 

 affected than the strong ones, the starved than the well fed, the 

 intemperate than the temperate, the fearful than the fearless ; but, 

 apart from these often doubtful distinctions, some other influences 

 must be at work in helping some to resist, for many a man or 

 woman of weak constitution has been able to pass through plagues 

 that had carried away more than one of powerful frame. This 

 resistance of some individuals to disease has probably at all times 

 attracted the attention of men, and very early in the history of 

 civilization observations have been made which by gradual exten- 

 sion have led to some of the most striking triumphs of medicine. 

 It will be my object in this lecture to show you how immunity to 

 disease, at first supposed to be due to individual peculiarities or 

 supernatural influences, has gradually become connected with cer- 

 tain external circumstances acting directly or indirectly. Among 

 the factors which are generally discussed in medical books as 

 influencing the liability or immunity of certain individuals to dis- 

 ease I may mention age, sex, family, and race. These, as far as 

 we can see at the present time, have an influence on the occur- 

 rence of disease which is in many instances difficult to explain. 

 Some facts, however, tend now and again to lighten our igno- 

 rance, and to show that even these apparently inherent qualities 

 are perhaps the result of the transmission of acquired properties 

 through generations of cells or of individuals. This will be more 

 evident perhaps if, by extending our field of observation from one 

 to several kinds, we consider how the immunity of certain species, 

 orders, or even classes of animals is brought about. Take, for 

 instance, the remarkable immunity of the fowl and of the frog to 

 anthrax. At first sight it seems impossible to understand why a 

 small animal like a frog or a fowl should be able to resist a dis- 

 ease that is so rapidly fatal to such large animals as the sheep, 

 man, or even the ox. Pasteur, however, more than twelve years 

 ago recognized that the difference of the body temperature of the 

 various animals was enough to affect the development of the par- 

 asite. He immersed a fowl for two days in water, bringing the 

 temperature down to 28 C, and showed that the fowl was as 

 liable to anthrax as any other animal. A similar observation was 

 made later on by another observer, who by raising the tempera- 

 ture of a frog rendered it also liable to the disease. Thus it was 

 demonstrated that certain conditions of temperature were neces- 

 sary for the anthrax bacillus to attain its full virulence. What 



