282 HEALTH AND DISEASE 



The cause may be purely mechanical, as in the case of a 

 blow which bruises tissue or breaks bone. It may be 

 chemical, as when a poisonous gas is inhaled or a poison 

 is eaten or drunk. Or it may be some one of the many 

 forms of disease-producing organisms which invade the 

 body and, if the conditions found there ,are favorable, 

 multiply rapidly as poisonous parasites. 



Microbes, a cause of disease. The living organisms 

 which are responsible for our infectious diseases, as 



Lactic acid V # ^ fr Pus 



(b) * ft 



Diphtheria / x \ 



(b > !\i> 



Tuberculosis ^> 



(b) \\\ N iVl r ( \\ From mouth 



Cholera OVV 1 * i vov / 



(b) '\'v V^ ^ } ^\\ From 



Influenza } 

 (b) *\ ; 



T r ^ 



ditch-water) 

 (s) 



Hay 

 (b) 



FIG. 151. Various forms of bacteria, highly magnified, some of which have flagella 

 or cilia: b, bacilli; m, micrococci; s, spirillae. 



consumption, cholera, typhoid fever, diphtheria and 

 malaria, come from either the vegetable or the animal 

 kingdom and are so small that they can be seen only 



