BACTERIA AND DISEASE 321 



chiefly, sometimes only, in the quotidian and malignant 

 types of malaria. 



(d ) The Flagellated Bodies apparently occur only in the 

 blood outside the body. They are extracorpuscular bodies, 

 and possess several long flagella, and are therefore actively 

 motile. They are derived from the crescents or irregular 

 intracorpuscular bodies. 



What is the precise significance of these various forms 

 and modifications of them is not at present known. Pos- 

 sibly the semilune is a resting stage inside the body, and 

 the flagellated body another similar stage outside. At- 

 tempts to cultivate the parasite outside the body have failed. 

 There is a good deal of evidence to show that the mosquito 

 is the host outside the human body. There may be differ- 

 ent forms and varieties of parasite, if not actually different 

 species, causing the diverse forms of clinical malaria. 



The above account of diseases caused by bacteria does 

 not profess to be in any sense exhaustive. It is merely 

 illustrative. It reveals some of the disease-producing pow- 

 ers of micro-organisms. There are a large number of other 

 diseases in which bacteria have been found. They are not 

 the causes, but only accidentally present or associated with 

 " secondary infection." Variola (small-pox), scarlet fever, 

 and measles are excellent examples. It is possible that the 

 danger at the present time is rather in the direction of sup- 

 posing that every disease will readily yield its secret to the 

 bacteriologist. Such, of course, is not the case. Never- 

 theless, as in the past, so in the future, constant research 

 and patient investigation is the only hope we have for the 

 elucidation of truth in respect to the causes of disease. 



