Introduction 3 



greater number of infective diseases are due to the development in 

 the organism of plants of the simplest structure, Bacteria. These 

 Bacteria produce the gravest and most destructive infections, such 

 as tuberculosis, bubonic plague, diphtheria, cholera, anthrax, the 

 pneumonias, suppuration, erysipelas, tetanus, glanders, leprosy, &c. 

 Among these bacteria some are too small to be resolved individually 

 under the highest magnifying powers and can only be made out en 

 masse. Such is the micro-organism of the contagious pleuro-pneumonia 

 of cattle. To this minuteness of certain pathogenic Bacteria is very 

 probably due the fact that in a considerable number of infections, 

 amongst which are scarlatina, measles, rabies, syphilis, aphthous fever 

 and small-pox, it has been impossible, up to the present, to recognise 

 any specific micro-organisms. 



It is probable that we shall succeed in discovering parasites, not 

 only in the diseases I have just cited, which present the characters of 

 infective and virulent diseases, but also in diseases of entirely different 

 types. In spite of the failure of various attempts to demonstrate the 

 parasite of malignant tumours, it may be hoped that, with improvement 

 in scientific methods, such a parasite will be unequivocally demon- 

 strated. In many other conditions which are at present considered 

 as not dependent on micro-organisms, an intimate connection with 

 such organisms will probably be established. Such are the atrophic 

 diseases and certain diseases of nutrition in which the parasites, with- 

 out playing a direct or immediate role, act by means of their products, 

 or by the changes which they set up in the affected organism. To 

 give an idea of this possibility it will be useful to cast a glance at the 

 various modes of action of the numerous etiological agents in infective 

 diseases. The parasites which produce them have, as a common 

 feature, their small dimensions ; they can only be recognised with 

 precision by the employment of high powers of the microscope. 

 They are likewise distinguished by a great variability, which is not 

 astonishing, since among infective agents are found on the one hand 

 animals of high structure (such as the Acari of itch) and on the other 

 plants of the simplest character such as the Gonococci or the various 

 Cocco-bacilli. 



The Acari are capable of perforating the epidermis by the mech- [4] 

 anical action of their feet and mouth-parts. They excavate channels 

 in the skin and thus provoke the irritation so characteristic of itch. 

 The larvae of the Trichinae in like manner produce marked lesions by 

 the mere mechanical act of penetration and migration in the striped 



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