TWENTY-FIVE YEARS OF BACTERIOLOGY 149 



properties of aggressive action or virulence, with which 

 is associated the ability to produce and liberate substances 

 paralyzing to the defensive processes of the host. Again, 

 the parasites may surround themselves with a kind of 

 mantle, protecting them from the potentially destructive 

 effects of serum and phagocyte. Or they may undergo an 

 internal change of constitution, through which resistance 

 to injurious agencies not normal to the species is de- 

 veloped. The last condition is called "fastness" and has 

 been observed especially among trypanosomes and spiro- 

 chetes exposed within the body of the host to ineffective 

 amounts of specific serums or chemicals. 



With so many factors interplaying, it is not difficult to 

 perceive that the problem of infection is a complex one, 

 both as regards its occurrence and its issue. But our 

 understanding of the conditions under which it arises has 

 been immeasurably extended by the discovery of the in- 

 sect and higher animal agencies in communicating infec- 

 tive agents to man, and of the part played by so-called 

 microbe carriers, those unfortunate and innocent persons 

 who have recovered from or merely been exposed to a 

 communicable disease, or suffered a slight, abortive, or 

 ambulant attack of which they are ignorant, and the dis- 

 covery of the usual portals of entry into the body of 

 pathogenic microorganisms. 



Infectious diseases prevail in two more or less distinct, 

 but at times interwoven ways, which we speak of as the 

 sporadic and the epidemic. The former represents the 

 ordinary manner of spread, the latter the occasional or 

 periodic explosive outbreak or wave, such as has been 

 experienced recently with the pandemics of poliomyelitis, 

 influenza and lethargic encephalitis. 



What has been sought in the past and is being assidu- 

 ously looked for in the present is an adequate explanation 

 of the transition from the sporadic to the epidemic type 

 of disease. We possess already quite accurate numerical 

 data which show the manner in which epidemics begin, 



