28 A STUDY IN HERE BIT V 



exclusively of disease, and chiefly of zymotic disease 

 — that is, disease due to the agency of those minute 

 living organisms known as microbes. Measles and 

 consumption are examples. In most countries 

 zymotic diseases are so prevalent that no man 

 escapes infection unless he be immune, nor death 

 unless he be resistant. 



Zymotic diseases may be divided into two 

 classes, which shade into each other. The first 

 class includes those diseases of which the microbes 

 have their habitation entirely or principally in the 

 human body ; the second class those of which the 

 microbes inhabit principally the environment outside 

 the human body, and to which a human prey is not 

 necessary. Contagious diseases are examples of 

 the first class ; their microbes inhabit wholly the 

 human body, being communicable by a sufferer to 

 another person by direct contact only. They are, 

 therefore, wholly parasitic, and parasitic on man 

 alone. Malaria is an example of the second class ; 

 a human prey is not necessary to the microbes, 

 which are abundant in many deserted and sparsely 

 inhabited tracts, and are therefore largely 

 saprophytic.^ Between the two extremes, between 



^ To avoid circumlocution, I use the word saprophytic, as meaning 

 merely that the microbes are capable of existence for an indefinite 

 period outside the human body. Properly speaking, a saprophyte 

 draws its nutriment from dead organic matter, a parasite from a living 

 being. Of course, therefore, the microbe of malaria is parasitic while 



