20 BACTERIOLOGY AND PROTOZOOLOGY 



these minute forms are present in or upon or have something 

 to do with the life of all the higher animals and plants. The 

 number of species in all is legion. The number of species 

 pathogenic for animals is but small. A microorganism is 

 pathogenic when it is capable of producing some form of 

 disease in the animal in which it is a parasite. In Chapter 

 III some of the known relations of non-pathogenic bacteria 

 will be discussed. It is sufficient here to emphasize the differ- 

 ence between the so-called parasites and saprophytes. 

 Parasites are organisms capable of living and multiplying 

 within the living animal body, sometimes to its detriment, 

 while saprophytes live on dead matter and may be found in 

 nature everywhere in air, soil, water. The body upon which 

 a parasite lives is called the host. There are a few of the 

 parasites that can carry on a saprophytic existence for a 

 short time (facultative parasites), while others (obligate 

 parasites), such as the organism of influenza, demand animal 

 juices for their nutriment. Among the protozoa this obligate 

 parasitism exists quite extensively, and many forms cannot 

 live at all if their normal cycle of life within the animal body 

 be disturbed. Indeed, we know the existence of many species 

 only because they pass through a certain development in 

 insects, then in higher animals and back again in insects; 

 that is, we only recognize them when they produce disease 

 (see Malaria). The saprophytes include the vast number 

 of organisms having important functions among the higher 

 vegetables and the growth of these in soil. It has been 

 suggested that at one time, now long past, all bacteria may 

 have been saprophytic. 



The general remarks concerning parasites apply alike to 

 protozoa and bacteria, but in medicine there is at the present 

 time more interest in the bacteria. For this reason only a 

 few diseases caused by protozoa are important. 



In order that the positions these unicellular forms occupy 

 in the living world may be known and used for reference to 

 large works their biological classification is given here. The 



