PARASITIC BACTERIA. 129 



the story which relates to the parasitic species. 

 These parasitic forms may do us direct or indi- 

 rect injury. But the species of bacteria which are 

 capable of doing us any injury, the pathogenic bac- 

 teria, are really very few compared to the great 

 host of species which are harmless. A small 

 number of species, perhaps a score or two, are 

 pathogenic, while a much larger number, amount- 

 ing to hundreds and perhaps thousands of species, 

 are perfectly harmless. This latter class do no in- 

 jury even though swallowed by man in thousands. 

 They are not parasitic, and are unable to grow in 

 the body of man. Their presence is entirely con- 

 sistent with the most perfect health, and, indeed, 

 there are some reasons for believing that they 

 are sometimes directly beneficial to health. It is 

 entirely unjust to condemn all bacteria because a 

 few chance to produce mischief. Bacteria in gen- 

 eral are agents for good rather than ill. 



There are, however, some species which cause 

 mankind much trouble by interfering in one way 

 or another with the normal processes of life. 

 These pathogenic bacteria, or disease germs, do 

 not all act alike, but bring about injury to man in 

 a number of different ways. We may recognise 

 two different classes among them, which, how- 

 ever, we shall see are connected by intermediate 

 types. These two classes are, first, the patho- 

 genic bacteria, which are not strictly parasitic but 

 live free in Nature ; and, second, those which live 

 as true parasites in the bodies of man or other ani- 

 mals. To understand the real relation of these 

 two classes, we must first notice the method by 

 which bacteria in general produce disease. 



