228 INTRODUCTION 



swine, etc. This point is more fully discussed in Chapter 

 XXIII, but cannot be too greatly insisted upon. 



"The greatest enemy to mankind is man." 



Exceptions to this statement do occur and are important, 

 and must be considered in efforts to protect completely 

 human beings from disease (tuberculosis from cattle, glan- 

 ders from horses, poisoning from spoiled canned goods, 

 anthrax from hair, hides, wool, of animals dead of the disease), 

 but the most common human diseases are derived from other 

 human beings directly or indirectly. 



Diseases which are due to unicellular pathogenic micro- 

 organisms are called infectious diseases, while if such diseases 

 are transmitted under natural conditions from organism to 

 organism they are spoken of as contagious diseases. Most 

 infectious diseases are contagious, but not all. Tetanus is 

 a good illustration of a non-contagious infectious disease. 

 There are very few such diseases. 



When a unicellular microorganism gains entrance into the 

 body and brings about any pathological changes there the 

 result is an infection. Undoubtedly many pathogenic organ- 

 isms get into the body but never manifest their presence by 

 causing disease conditions, hence do not cause an infection. 

 It is the pathological conditions which result that constitute 

 the infection and not the mere invasion. 



The time that elapses between the entrance of the organ- 

 ism and the appearance of symptoms is called the period of 

 incubation and varies greatly in different diseases. 



The term infestation is used to denote pathological con- 

 ditions due to multicellular parasites. Thus an animal is 

 infested (not infected) with tapeworms, roundworms, lice, 

 mites, etc. Many of these conditions, probably all, are 

 contagious, i. e., transmissible naturally from animal to 

 animal. The word contagious has been used in a variety 

 of ways to mean communicated by direct contact, communi- 

 cated by a living something {contagium) that might be car- 

 ried to a distance and finally communicable in any manner, 

 transmissible. The agency of transmission may be very 



