30 IMMUNOLOGY 



of the plant, while the bacteria are supplied certain food materials 

 by the host. This relationship of mutual helpfulness is spoken 

 of as symbiosis. 



Infection Without Pathological Change. — A generalized in- 

 fection of the wild rat witli Tnjponosome leivesi or with Leptospira 

 icterohemorrhagiae is not beneficial to tlie host but apparently 

 does not elicit any physiological or anatomical disturbances that 

 can be designated as infectious disease. In both of these examples 

 of infection the organisms enjoy a wide distribution in the body 

 and are present in great numbers in the blood. 



Infection With PathologicaI: Change. — These examples of 

 infection without pathological change or infectious disease are in 

 nuirkod contrast to what occurs when the Icptospira, mentioned 

 al)ove, gains entrance to tlie tissues of another rodent, the guinea 

 pig, or for that matter into the tissues of man. In either case the 

 presence of generalized infection causes profound pliysiological 

 and structural disturbances, frequently resulting in death. 



Types of Pathological Changes. — It would seem that if an 

 infectious agent is to produce disease, it must not only gain 

 entrance to the tissues and be able to live and multiply there, but 

 it or its products of growth must be incompatible with the normal 

 physiological functioning of the host and lead to discernible patho- 

 logical (abnormal) changes. These may be local, focal, or general. 

 For example, organisms gaining entrance to the body are fre- 

 quently "filtered out" by the regional lymph glands. This maj^ 

 occur without any symptoms other than a slight increase in size 

 and change in consistency of the glands. The organisms may be 

 destroyed or remain quiescent for long periods of time or until 

 conditions become favorable for their growth and dissemination. 



Localized and Generalized Infection. — A wound may become 

 infected and show abnormal changes such as redness, swelling, 

 local heat and pain, without general physiological symptoms. As 

 long as the infectious agents remain at the site of injury, it is a 

 localized infection, but when they multiply and invade the blood 

 stream, it becomes a generalized infection. This usually calls forth 

 definite symptoms even before the blood stream is invaded. 



Bacteriemia, Septicemia, and Pyemia. — If bacteria gain en- 

 trance to the blood stream from some focus but are unable to 

 multiply in the circulation, the condition is spoken of as a 



