Minnesota Plant Life. 107 



form. An infections disease is caused by a germ wliicli may feed 

 and develop in tlie body of the animal, but habitually feeds and 

 develops in not-living substances, such as sewage, river-water 

 or the soil. Both varieties of germ are alike in one respect, that 

 they poison the body, for they secrete into it certain substances 

 known under the general name of ptomaines, comparable as has 

 been before said, to snake, alkaloid or albuminoid poisons. An 

 invasive disease is produced by the development, in the body, of 

 a germ which though not violently poisonous yet causes disease 

 through the multiplication of the bacterium itself. Smallpox is 

 an example of a contagious, typhoid fever of an infectious, and 

 lumpy-jaw in cattle of an invasive disease, though the germ 

 which causes the latter is scarcely a true bacterium. The cow 

 with lumpy-jaw has the tongue and jaw leavened by the germ 

 much as though they were loaves of bread, but the animal is not 

 seriously poisoned and does not generally perish as quickly as 

 does an animal when attacked by some fatal infectious or con- 

 tagious disease like Texas fever, anthrax or hog-cholera. 



Quarantine and sanitation. It is apparent that between con- 

 tagious and infectious diseases — the one produced by what is 

 called an obligatory parasite and the other by what is termed a 

 facultative parasite — demand widely different hygienic methods 

 of prevention. Against contagious diseases quarantine is the 

 great preventive, for if the person who is diseased can be kept 

 by himself others will not be aiTected. Against infectious dis- 

 ease hygiene is the great preventive, including here especially 

 what is known as sanitation. When one knows that dangerous 

 germs such as those of typhoid fever may develop and multiply 

 in garbage heaps, in sewage, in unfiltered river water, in neg- 

 lected reservoirs, in badly prepared foods and stale fruits, he 

 understands the importance of removing his garbage, attending 

 to his sewage, boiling his water and taking heed to his kitchen, 

 his ice-box and his diet. All the methods of modern quaran- 

 tine and sanitation are intelligently prosecuted in the light of 

 an increasing knowledge of bacterial lives and habits. 



Inoculation and vaccination. Passing from simple preven- 

 tive measures to somewhat more complicated procedure it be- 

 comes necessary to consider the processes known as inoculation 

 and vaccination. Inocnlation is a name given to the intentional 

 and actual transference of the germs of a disease to the body, 



