INFECTIOUS DISEASES AND MICROBES, ETC. 179 



being conveyed to distant points by the instru- 

 mentality 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 

 jiow that diphtheria, like typhoid fever or scarlet 

 fever, can be, and, as a matter of fact is, often con- 

 veyed from an infected source to great distances by 

 the instrumentality of milk. In malignant anthrax, 

 another disease in which the contagium is convey- 

 able by direct contact, e.g. in the case of an abrasion 

 or wound on the skin coming in contact with the 

 blood of an animal dead of anthrax, we know that 

 the spores of the anthrax bacilli can be, and, in 

 many instances are, conveyed to an animal or a 

 human being by the air, water, or food. The 

 bacilli of tubercle, finding entrance through a 

 superficial wound in the skin or mucous membrane, 

 or through ingestion of food, or through the air, can 

 in a susceptible human being or an animal produce 

 tuberculosis either locally or generally. The differ- 

 ence as regards mode of spread between different 

 diseases resolves itself merely into the question, 

 Which is, under natural conditions, the most 

 common mode of entry of the disease-germ into 

 the new host? In one set of cases, e.g. typhoid 

 fever, cholera, the portal by which the disease-germ 

 generally enters is the alimentary canal ; in another 

 set an abrasion or wound of the skin is the portal, 

 as in hydrophobia, tetanus, and septicsemia ; in 

 another set the respiratory organs, or perhaps the 

 alimentary canal, or both, are the paths of entrance 



