IO 6 Minnesota Plant Life. 



await investigation. But of the disease-producing group there 

 are some with which men are unfortunately only too familiar. I 

 may name three of especial interest, and valuable in the illus- 

 tration of different points. The diseases known as consump- 

 tion or tuberculosis, typhoid fever and smallpox are certainly 

 produced in the human body by three distinct species of bac- 

 teria, known respectively as consumption-bacteria, typhus- 

 bacteria and smallpox-bacteria. The contraction of any of 

 these diseases does not necessarily follow the entrance into the 

 body of the bacteria, for, in the first place the bacteria may 

 enter in a weak, non-virulent condition; in the second place, 

 they may be destroyed in the body in a great variety of ways 

 before they can develop and multiply sufficiently to do any 

 harm ; in the third place, they may find the body in a state which 

 would prevent their nourishment so that they would die of 

 their own accord. It is probably true that men frequently take 

 into their bodies harmful germs, which, under the beautiful 

 police-system of the blood, are snapped up by the white blood- 

 corpuscles and destroyed before they can accomplish any mis- 

 chief by their poison-secreting habits. That condition of the 

 body, in whatever way it should be explained, under which bac- 

 teria of a certain disease cannot grow, is called immunity, and 

 immunity may be apparently either inherited or acquired. Im- 

 munity to yellow fever seems to be inherited by certain classes 

 of people living in the tropics, but it may be acquired by persons 

 born outside the tropics after they have passed through the ex- 

 perience of acclimatization. 



Contagious, infectious and invasive diseases. If, however, 

 one is not immune to consumption, typhus or smallpox, and the 

 germs of one of these diseases enter his system, and multiply too 

 rapidly for the white blood-corpuscles to destroy them, then 

 very soon what are termed the symptoms of. the disease begin 

 to appear and the patient has the disease, or rather, the disease 

 has him, for he is now the prey of an invading force of tiny fun- 

 gus parasites. Here a distinction must be made between three 

 types of bacterial disease which affect animals, and are known 

 as contagious, infectious and invasive diseases. A contagious 

 disease is caused by a germ which can ordinarily live only in the 

 body of the diseased animal or in that of some closely related 



