214 INTRODUCTION 



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 inmsion. 



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 condi- 

 tions 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., transmissable 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 

 roundabout — as through a special tick in Texas fever, a 

 mosquito in malaria, etc., or by direct personal contact, as 

 generally in venereal diseases. After all, though exactness 

 is necessary, it is better to learn all possible about the means 

 of transmission of diseases, than quibble as to the terms to 

 be used. 



An infectious disease may be acute or chronic. An acute 

 infection is one which runs for a relatively short time and is 

 "self-limited," so-called, i. e., the organisms cease to mani- 

 fest their presence after a time. In some acute infections 

 the time is very short — German measles usually runs five 

 or six days. Typhoid fever may continue eight to ten 

 weeks, sometimes longer, yet it is an acute infectious dis- 

 ease. It is not so much the time as the fact of self-limita- 

 tion that characterizes acute infections. 



