1891.] on Infectious Diseases. 281 



known as lupus — at any rate some forms of it — and further the 

 disease scrofula, so often present in children, are really of the nature 

 of tuberculosis, the former in the skin, the latter in the lymph 

 glands. 



Now see what an enormous step in advance this constitutes : — 



(1) We can now diagnose tuberculosis with much greater accuracy 

 in man and animals, even in cases in which this was formerly difficult 

 or impossible. 



(2) We have accepted rightly that all forms of tuberculosis are 

 infectious or communicable diseases, communicable by inoculation, 

 by ingestion, i.e. by food, or by respiration, i.e. by air. 



(3) We have learned to recognise that, as in other infectious 

 disorders, there exists a risk to those suscejptible to tuberculosis, of 

 contracting the disease from a tubercular source, and it is the recog- 

 nition of these facts which ought to regulate all efforts to prevent its 

 spread. 



Tetanus or lockjaw, not previously known to be so, has likewise 

 been fully demonstrated to be an infectious disease : we now know 

 that it is due to a bacillus having its natural habitat in certain garden 

 earth ; that this bacillus forms spores, that these spores gaining access 

 to an abrasion or wound of the skin in man or animals are capable of 

 germinating there and multiplying, and of producing a chemical 

 poison which is absorbed into the system, and sets up the acute 

 complex nervous disorder called lockjaw. The recognition of the 

 disease as an infectious disease and caused by a specific microbe has 

 taught us at the same time the manner in which the disease is con- 

 tracted, and thereby the way in which the disease is preventable* 

 [2. Demonstration : lantern slides of tubercle and tetanus.] 

 The study of disease germs by the new and accurate methods of 

 bacteriology has also led to a clearer and better understanding of the 

 manner in which at any rate some of the infectious diseases spread. 

 While it was understood previous to the identification of their precise 

 cause that some spread directly from individual to individual (e. g. 

 small-pox, scarlet fever, diphtheria), others were known to be capable 

 of being conveyed from one individual to another indirectly, i. e. 

 through adhering to dust, or being conveyed by water, milk, or by 

 food-stuffs (e. g. cholera, typhoid fever). But we are now in a posi- 

 tion to define and demonstrate more accurately the mode in which 

 infection can and does take place in many of the infectious diseases. 

 By these means we have learned to recognise that the popular dis- 

 tinction between strictly contagious and strictly infectious diseases — 

 the former comprising those diseases which spread as it were only by 

 contact with a diseased individual, while in the latter diseases no 

 direct contact is required in order to produce infection, the disease 

 being conveyed to distant points by the instrumentality of air, water, 

 or food — is only to a very small extent correct. Take, for instance, 

 a disease like diphtheria, which was formerly considered a good 

 example of a strictly contagious disorder ; we know now that diph- 



