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TROPICAL DISEASES 



man and animals, as they did when hosts were first evolved, only 

 the matter is more complicated owing to the evolution of methods 

 of defence upon the part of the host. 



In his most interesting and masterly book Adami has shown that 

 a non-pathogenic organism can be made pathogenic by injecting 

 into an animal killed non-pathogenic bacilli, and then ten or fifteen 

 days later the live bacilli. These, when recovered from the tissues 

 of the experimental animals, were found to have undergone consider- 

 able variation due to their altered environment, and had so changed 

 their characters as to be very virulent to guinea-pigs. In other 

 words, by means of a preliminary anaphylactic phenomenon a non- 

 pathogenic microbe may become pathogenic and a new disease be 

 evolved. 



Similarly, according to some of our observations, when man is 

 immunized artificially against typhoid and the paratyphoids, he 

 becomes more susceptible to infections by chance intestinal organ- 

 isms, which in this way give rise to new diseases. 



Finally a germ accustomed to a human race which has developed 

 a certain amount of immunity may meet with another race which 

 lacks this immunity, and immediately a disease which is mild may 

 develop rapidly fatal attacks and spread widely — e.g., measles in 

 the Sandwich Islanders in London long ago. 



Enough has been said to show the line of thought — viz., the 

 importance of environment, the difficulty of impressing characters, 

 but the hereditary transmission of these characters when once 

 evolved producing variation in the parasites, while the reaction on 

 the part of the host tending to produce an immunity against older 

 forms, and yet leaving, in the phenomenon of anaphylaxis, a gate 

 open for further variation on the part of non-pathogenic organisms 

 and the possibility of the origin of new diseases. 



GEOGRAPHICAL DISCOVERY. 



Just above we have mentioned the introduction of pathogenic 

 organisms to races to which they were previously unknown, and as 

 this is primarily due to geographical discovery, we desire to invite 

 the reader's attention to this factor in the dissemination of tropical 

 disease, as the discovery of new lands has eventually led to the 

 betterment of means of communication, and hence to the easier 

 and quicker transference of the microbes of disease. 



Diseases which may have been endemic in one region for centuries 

 may, by means of geographical discovery and by means of the present 

 and past wars, spread to other regions where they were previously 

 unknown ; and this brings us to the consideration of endemicity and 

 epidemicity. 



ENDEMICITY. 



As so much epidemiological investigation of disease has of late 

 been undertaken, mainly due to the initiative of Manson and Sambon, 



