CHAPTER XXI 



DISEASE, INFECTION, AND RESISTANCE 



Disease. Disease may be defined as any abnormality 

 in form or functioning of the body or of any of its parts. 

 Thus denned the term includes many conditions frequently 

 not thought of as disease. It includes, for example, condi- 

 tions usually termed traumata, that is, the changes resulting 

 from mechanical injury, such as a blow, a bruise or a cut. 

 In addition to the traumatic diseases there are so-called 

 dietary or deficiency diseases. These are known both in 

 plants and in animals. A lack of sufficient iron, for ex- 

 ample, will, in certain plants, result in a whitening of the 

 leaves, that is, in a decrease in the amount of chlorophyl 

 present. In man and animals diseases sometimes result 

 from an improper balance among the principal food con- 

 stituents: fats, proteins and carbohydrates. Still more 

 frequently occur those diseases due to the lack of certain 

 so-called accessory substances or vitamines. Such, for 

 example, are the diseases scurvy, beriberi and probably 

 pellagra. Still another group of diseases is due to the 

 improper functioning of certain glands of the body such as 

 goitre and diabetes. There may be also hereditary defects 

 such as color blindness or congenital defects such as hare 

 lip or clubfoot. All the conditions enumerated may be 

 grouped together as noninfectious diseases. 



An infectious disease is one caused by some microorgan- 

 ism. It is evident, therefore, that the primary grouping of 

 diseases may be made into infectious and noninfectious. 



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