HOW DISEASES ARE CAUSED 



303 



some cases certain of the substances thus excreted are poisonous 

 to the host. In this way the host may be made seriously ill or 

 even be killed by the poison. Diseases known to be of this kind 

 in man number between thirty and forty, and it is possible that 

 still other ailments may be due to such poisoning. The para- 

 sites causing such disturbances may be plants (mostly bac- 

 teria) or they may be animals (chiefly protozoa). Among the 



Fig. 145. The effects of a plant parasite 



A healthy cotton plant and one attacked by xvilt, a diseased condition due to the 

 action of a microscopic parasite related to the molds. (From Duggar's "Fungous 



Diseases of Plants"') 



diseases caused in this way by bacteria are diphtheria, typhoid 

 fever, and tetanus, or lockjaw. Among the diseases caused in 

 this way by parasitic protozoa are malaria, dysentery, and 

 sleeping sickness. 



A third effect that parasites may produce upon the host is 

 that of destroying special tissues or organs by using the mate- 

 rial as food (see Fig. 145). The name consumption, which was 

 formerly very common for tuberculosis of the lungs, tells this 

 story strikingly. Parasites that injure chiefly by destroying tis- 

 sues may also give off poisons to some degree, and vice versa. 



