6 INFECTIOUS AND PARASITIC DISEASES. 



ogists discovered that a number of the infectious 

 diseases were due to bacteria and, rather hastily, it 

 must be confessed, concluded that all of this class had 

 a similar origin. So they described an infectious 

 disease as ''a morbid or diseased state of the body due 

 to the invasion and growth of bacteria." Somewhat 

 later it was seen that they had gone too far in formulat- 

 ing this definition for infectious diseases, because a 

 certain number were discovered which owe their origin 

 to animate agents that are not bacteria, namely, to 

 moulds and protozoa.* 



Hence it has followed that the designation ''infectious 

 disease" is restricted in its application to diseases which 

 conform to a certain type, and ''infectious agent" to 

 a living organism, | microscopic in size, that is capable 

 of producing an infectious disease. So far as we know, 

 only three types of organisms produce infectious diseases, 

 namely, bacteria, moulds and protozoa, certain species 

 of each comprising the infectious agents. At present 

 bacteria constitute the vast majority, but it is fast 



* Protozoa (literally, first animal) are the simplest organisms which 

 clearly belong to the animal kingdom. The characteristic which dis- 

 tinguishes them from all other groups of animals is the fact that each 

 protozoan consists of a single cell. 



t Organism — composed of organs functionally necessary for the ex- 

 istence of the individual or race. Although unicellular plants and 

 animals have no organs, yet, since they perform functions analogous to 

 those of the higher forms of living matter, we are correct in considering 

 them organisms. 



