io INFECTION AND IMMUNITY 



simple in structure, consisting, as a rule, of a single 

 cell, they may be differentiated by the expert without 

 great difficulty, and classified as animal micro-organ- 

 isms ("Protozoa") or as vegetable micro-organisms 

 (" Protophyta "). By far the greater number of 

 known disease germs are recognised as vegetable 

 micro-organisms belonging to the class known as 

 Bacteria. This class includes a large number of 

 harmless species, which abound especially in surface 

 waters and in the upper layers of the soil. 



The bacteria are classified with reference to their 

 form. Those which are spherical are called micro- 

 cocci / those which are longer in one diameter than 

 in the other oval, rod-shaped, or filamentous are 

 called bacilli ; those which are elongated and spiral 

 in form, like a corkscrew, are called spirilla (singu- 

 lar micrococcus, bacillus, spirillum). The germs of 

 pneumonia, of erysipelas, of boils and abscesses, of 

 Malta fever, of cerebro-spinal meningitis, and some 

 others are micrococci, all having distinct specific char- 

 acters by which they can readily be recognised by an 

 expert bacteriologist. The germs of typhoid fever, 

 of tuberculosis, of influenza, of diphtheria, of dysen- 

 tery, of bubonic plague, of tetanus, and of several 

 infectious diseases of the lower animals (hog cholera, 

 swine plague, anthrax, glanders) are bacilli. As in 

 the case of the pathogenic micrococci, these all have 



