BACTERIA AND PROTOZOA. 15 



history of the former is simpler, and they multi- 

 ply only by fission (fission fungi). Forms occur 

 which appear to be intermediate between the true 

 fungi and the true bacteria. The protozoa, the 

 lowest forms of animal life, vary greatly in form 

 and in the complexity of their life cycles. The 

 highest protozoa lead an intricate existence in 

 which sexuality and alternation of hosts are some- 

 times conspicuous features, as in the case of the 

 parasites of malarial fevers, and possibly in that of 

 yellow fever. In some instances the alternation 

 of hosts is purety a facultative property, and not a 

 necessity for the perpetuation of the species, al- 

 though it is part of the natural cycle; this is the 

 case with some of the trypanosomes, which can be 

 transferred from animal to animal artificially for 

 an indefinite period. 



There are certain infections, the causes of which 

 are not known, and in some instances the organ- 

 isms are considered as ultramicroscopic because 

 they pass through bacteriologic filters. Theoretic- 

 ally they may be either bacteria or protozoa. 



The known pathogenic micro-organisms may Types of MI- 

 also be placed in one of the following three groups : 

 1. Obligate parasites, which are capable of growth 

 only in a living organism (the bacillus of leprosy 

 and the organisms of malaria). 2. Facultative 

 saprophytes, which usually exist as parasites but 

 may multiply on inanimate material under proper 

 conditions. This group includes f most of the path- 

 ogenic microparasites. 3. Facultative parasites, 

 which are saprophytic organisms living readily on 



inanimate material, but which may produce dis- 

 ease when they reach suitable tissues in a host 



