700 BEroRT— 188S. 



It was, of course, clear that contagion must be due to the communication of in- 

 fectious particles or contagia. These contagia, although at the time no one had seen 

 them, Henle pointed out, ' have the power, possessed, as far as we know, by li-viog 

 creatures only, of growing under favourable conditions, and of multiplying at the 

 expense of some other substance than their own, and therefore of assimilating that 

 substance.' Henle enforced his view by comparison with the theory of fermenta- 

 tion, which had then been promulgated by Schwann. But for many years his views 

 found no favour. Botanists, however, as in so many other cases, struck on the 

 right path, and from about the year 1850 steady progress, in which De Bary him- 

 self took a leading part, was made in showing that most of the diseases of plants 

 are due to parasitic infection. The reason of this success was obvious : the structure 

 of plants make them more accessible to research, and the invading parasites are 

 larger than animal contagia. On the animal side all real progress dates from about 

 1860, when Pasteur, having established Schwann's theory of fermentation on an 

 impregnable basis, took up Henle's theory of living contagia. 



The only risk now is that we may get on too fast. To put the true theory of 

 any one contagious disease on as firm a basis as that of alcoholic fermentation is no 

 easy matter to accomplish. But I believe that this is, notwithstanding a flood of 

 facile speculation and imperfect research, slowly being done. 



There are two tracts in the body which are obviously accessible to such minute 

 organisms as bacteria, and favourable for their development. These are the ali- 

 mentary canal and the blood. In the case of the former there is evidence that 

 every one of us possesses quite a little flora of varied forms and species. They 

 seem for the most part, in health, to be comparatively innocuous; indeed, it is be- 

 lieved that they are ancillary to and aid digestion. But it is easy to see that other 

 kinds may be introduced, or those already present may be called into abnormal 

 activity, and fermentative processes may be set up of a very inconvenient kind. 

 These may result in mere digestive disorder, or in the production of some of those 

 poisonous derivatives of proteids of which I have spoken, the eflect of which upon 

 the organism may be most disastrous. 



The access of bacteria to the blood is a far more serious matter. They produce 

 phenomena the obvious analogy of which to fermentative processes has led to the 

 resulting diseases being called zymotic. Take, for example, the disease known as 

 ' relapsing fever.' This is contagious. After a period of incubation, violent fever 

 sets in, which lasts for something less than a week, is then followed by a period of 

 absence, to be again followed in succession by one or more similar attacks, which 

 ultimately cease. Now you will observe that the analogy to a fermentative process 

 is very close. The period of incubation is the necessary interval between the 

 introduction of the germ and its vegetative multiplication in sufficient numbers to 

 appreciably affect the total volume of the blood. The rise in temperature and the 

 limited duration of the attack are equally, as we have seen, characteristic of 

 fermentative processes, while the bodily exhaustion which always follows fever is 

 the obvious result of the dissipation by the ferment organisms of nutritive matter 

 destined for the repair of tissue-waste. During the presence of this fever there 

 is present in the blood an organism, Sjnrochcete Ohenneieri, so named after its 

 discoverer. This disappears when the fever subsides. It is found that if other 

 individuals are inoculated with blood taken from patients during the fever attack, 

 the disease is communicated, but that this is not the case if the inoculation is made 

 during the period of freedom. The evidence then seems clear that.tliis disease is 

 due to a definite organism. The interesting point, however, arises, why does the 

 fever recur, and why eventually cease ? The analogy of fermentation leads to the 

 hypothesis that, as in the case of yeast, the products of its action inhibit after a 

 lime the further activity of the Spirochete. The inhibiting substance is, no doubt, 

 eventually removed partially from the blood by its normal processes of depuration, 

 and the surviving individuals of Spirocheete can then continue their activity, as in 

 lactic fermentation. With regard to the final cessation of the disease, there are 

 facts which may lead one to suppose that in this as in other cases sufficient of the 

 inhibiting substance ultimately remains in the organism to protect it against any 

 further outbreak of activity on the part of the Spirochete, 



