530 Transactions of the Society. 



tricha, many members of these for long being regarded as true 

 bacteria, whereas we now know them to be parasitic fungi of the 

 hyphomycetic type. 



Now, the true botanical position of these organisms, such, for 

 example, as the bacillus of Koch, was primarily arrived at by 

 prolonged morphological study, a branch of bacteriological observa- 

 tion which in these days is in some danger of being neglected, 

 owing to the counter-attractions presented by study of " anti- 

 bodies " in test-tubes. And this neglect of " bacterial " mor- 

 phology has, I think, had this unfortunate result, that it has led 

 many a worker to assume that perfectly genuine examples of 

 pleomorphic activity are merely examples of contamination, either 

 by extraneous organisms or by secondary invaders. By this I 

 mean that cultures are irequently rejected because they appear to 

 be contaminated, when in reality they are often pure cultures, 

 not, as supposed, of bacteria, but of parasitic fungi in the botanical 

 sense. In any case of doubt it is, of course, always necessary, 

 when visible organisms are being studied, to cultivate from single 

 individuals ; but even then it is often useless to be content with 

 cultivation on standardized laboratory media only. But even in tlie 

 case of growth from single colonies it is most dangerous to assume 

 contamination from the mere fact of pleomorphism, for the simple 

 reason that it may prove that we are here dealing, not, as supposed, 

 with bacteria, but with a fungus, or other organism, with a complex 

 life-history which may demand, and often does demand, special 

 methods of attention to make it reveal its true nature. If, in fact, 

 a good technique has been observed, the very occmTence of pleo- 

 morphism, especially if repeatedly confirmed, is strong presumptive 

 evidence that we have before us not a bacterium but a fungus, or 

 other unicellular organism with a complex life-history. And, as 

 you will see in the case of typhus fever, the additional work 

 involved by realizing this possibility becomes amply justified. 



In the study of infective disease in man, this question of con- 

 tamination is again and again being brought up, and in the study 

 of those of the exanthemata to which I have specially addressed 

 myself — uamely, to cerebrospinal fever, typhus fever, scarlet fever, 

 measles, typhoid fever, and paratyphoid fever — this question of 

 careful scrutiny of alleged contamination is absolutely fundamental. 

 In all these diseases, as well as in many others, one organism 

 peculiar to each disease repeatedly appears : the meningococcus of 

 Weichselbaum in cerebrospinal fever ; the Diplobacillus e.xanthe- 

 maticus iu typhus fever ; a streptococcus of sorts in scarlet fever ; 

 the bacillus of Eberth in typhoid fever ; the Bacillus paratypJwsus A 

 in one form of paratyphoid fever, and so on. In each of these 

 diseases the particular organism appears with remarkable con- 

 stancy, and, speaking generally, in no others. In each case the 

 particular organism is believed by its discoverer to be the cause of 



