1919] on A Filter-passing Virus in Certain Diseases 551 



particular malady. Discoveries and facts of this nature have often 

 explained the mysterv of the apparent spontaneous origin of diseases 

 at certain places at certain times, when there is no evidence of 

 importation of actual cases. 



Sufficient has probably been said to emphasize the importance, if 

 medicine is to be truly scientific in its methods, for the causes of 

 disease to be studied as fully as possible and placed on a sure founda- 

 tion. Great progress in this direction has taken place during the 

 last seventy years, and especially during the last fifty years ; but 

 modern investigators, with all the means at present available at 

 their disposal, cannot fail to be profoundly impressed with the 

 foreshadowing of results often arrived at by the fathers of medicine. 



Speaking broadly, the great majority of diseases are due to the 

 direct action of one or more of the following causes : (1) physical 

 agents ; (2) chemical agents ; (3) living agents, or viruses. In the 

 past much stress was laid on the first group, as, for instance, the 

 influence of cold as a direct cause of disease. Even as recently as 

 thirty years ago exposure to cold was regarded as the main cause of 

 a great number of diseases of various organs of the body, such as 

 the lungs, heart, kidneys, spinal cord, etc., and not only so, but the 

 a-nomenclature was introduced, and still obtains, suggesting that these 

 affections were actual inflammations, due probably to extreme con- 

 gestion of the internal parts produced by the direct action of the 

 external cold. Modern investigation has shown that in most of these 

 diseases a living virus was the effective cause, and that cold, if it had 

 any action at all, only acted as a predisposing cause. Hence, at 

 the present day, it is only in a very limited class of diseases that we 

 recognise physical agents such as heat, cold, changes of pressure, as 

 true effective causes. 



There can be no question that some diseases are due to chemical 

 agencies, and here maladies may be produced in one of two ways : 

 (i.) by the direct action of some poisonous material ; or (ii.) so-called 

 deficiency diseases, where pathological changes result from the with- 

 holding of some substance essential to the wellbeing of the body. 



Most diseases of man and animals where the cause is known, 

 are dependent upon the action of a living virus, and thus the study 

 of disease is really a branch of Biology, and strictly a phenomenon 

 of Parasitism. The known pathogenic viruses belong some to 

 the vegetable kingdom, some to the animal, and some are not as 

 yet sufficiently known to make their classification at all certain. 

 Some of the most important diseases, especially perhaps those of 

 tropical regions, are due to animal parasites either protozoal or of 

 the higher phyla of the animal world. The diseases of vegetable 

 origin are mostly due to bacteria ; but yeasts and fungi are also 

 factors of some importance. 



The bacteria, or micro-organisms, are very numerous, responsible 

 for very many diseases, and the greatest triumphs in medical science 



YoL. XXII. (No. 113) 2 p 



