HOW DISEASES ARE CAUSED 305 



(1822-1895) and others as to the relation between microbes 

 and disease. For the first time we knew exactly how^ certain 

 particular diseases were caused, and with this knowledge it 

 has become possible in many cases to cure with greater cer- 

 tainty than ever. But what is more important, we are now 

 able systematically to prevent diseases that formerly destroyed 

 millions of people. 



HOW DISEASES ARE CAUSED 



1. Food relations of organisms 



Independent organisms (chlorophyl bearers) 



Organisms living on dead organic materials 



Organisms killing others for food {predatory) 



Organisms living at the expense of other organisms {parasites) 



Complete or partial parasitism 



Permanent or temporary parasitism 



Obligatory or optional parasitism 

 Organisms exchanging materials or services with other organisms 

 {mutualism) 



2. Effects of parasites upon hosts 



Withdrawing material 

 Poisoning with by-products 

 Destroying tissues 



3. How organisms may become diseased 



Injuries caused by parasites Nutritional deficiencies 



Poisons Vitamin A (rickets) 



Spoiled food Vitamin B (beriberi) 



Unsuitable material used as food Vitamin C (scurvy) 



Inorganic poisons lodin (goiter) 



Lead Unknown causes 



Mercury Cancer 



Copper Diabetes 



Phosphorus Bright's disease 



4. Effects of disease (symptoms) 



Excessive activity in some organs or tissues 



Deficient activity in some organs or tissues 



Destruction of some tissues or organs 



Pains and discomforts 



Chemical changes in various fluids of the body 



