14 INFECTION AND IMMUNITY. 



There is another large class of parasites, how- 

 ever, which under proper conditions cause severe 

 diseases in the host. Many pathogenic microbes live 

 in and on the skin and mucous membranes with- 

 out doing harm, but if certain ones reach the 

 deeper tissues, they may institute pathologic proc- 

 esses (e. g., staphylococci, streptococci, pneumococ- 

 ci, diphtheria bacilli and meningococci). Any or- 

 ganism which is able to cause pathologic tissue 

 changes, to disturb functions, and to set up ab- 

 normal symptoms is classed as a pathogenic para- 

 site. The abnormal processes which they institute 

 are our infectious diseases. 



infestation Where certain comparatively large organisms 

 I *tion" (macroparasites) exist on a body surface, as the 

 skin or intestinal tract, the surface is said to be 

 infested; the skin, for example, is infested with 

 pediculi. One may also say that the intestinal 

 tract is infested with tapeworms, but here the dis- 

 tinction between infestation and infection is not to 

 be drawn so sharply; surely when the larvae pene- 

 trate the intestinal wall and reach the circulation 

 or distant organs we must speak of infection. But 

 even the adult tenia as it exists in the intestines 

 may cause erosions of the mucous membrane or 

 may perhaps burrow a slight distance into the 

 wall, a condition which approximates the action of 

 the larvae in passing through the wall ; accordingly 

 at some point the distinction between infestation 

 and infection becomes an arbitrary one. 



Bacteria and The known pathogenic micro-organisms are 

 protozoa. grouped among the fungi, the bacteria and the pro- 



tozoa. Both the bacteria and fungi are vegetable 

 in nature, complexity in form and methods of 

 growth characterizing the latter, whereas the life 



