METHODS AND CHANNELS OF INFECTION. 521 



MICROORGANISMS OF DISEASE CONSIDERED AND CLASSIFIED. 



PATHOGENIC BACTERIA. Bacteria which produce diseases are known 

 as pathogenic bacteria. Of the many thousand species of bacteria only 

 a comparatively few species have anything to do with the pathogenic 

 processes in the plant or animal body. Some of these pathogenic 

 bacteria can grow only in the animal or plant body, and do not exist 

 outside of it; for this reason they are known as obligate parasites. There 

 are still others which produce disease in the animal body which can grow 

 and reproduce outside. They are known as facultative saprophytes. There 

 are still other bacteria which ordinarily live outside the animal and plant 

 body and exist largely upon dead organic material. These are known to 

 occasionally produce pathogenic processes in the body. They are called 

 facultative parasites. As examples of obligate parasites we might mention 

 the spirillum of relapsing fever. As an example of facultative sapro- 

 phytes may be mentioned B. typhosus, of typhoid fever. As examples 

 of facultative parasites may be mentioned B. tetani of tetanus, and Bact. 

 welchii. 



PATHOGENIC PROTOZOA. There are several infectious diseases in man 

 and animals which are caused by pathogenic protozoa. Among the 

 common ones may be mentioned malaria, amoebic dysentery, Texas 

 fever, infectious jaundice of dogs, and the various trypanosome diseases, 

 such as sleeping sickness, nagana, dourine, mal de caderas. With a few 

 exceptions it is impossible to cultivate artificially these protozoa outside the 

 animal body. The Trypanosoma brucei, of nagana, has been cultivated 

 as well as the Trypanosoma lewisi of the rat. The Amoeba coli of 

 dysentery has also been cultivated, and it is stated that under certain con- 

 ditions Babesia bigemina of Texas fever may be artificially grown. 



ULTRAMICROSCOPIC MICROORGANISMS OR VIRUSES. There are some 

 infectious diseases the causes of which have never been discovered. The 

 infectious agents cannot be cultivated and cannot be stained by the 

 ordinary bacteriological methods (p. 64). There are several infectious 

 diseases of unknown cause, the viruses of which do not act in accordance 

 with the tests of ultramicroscopic or filterable viruses, for example 

 smallpox, cowpox and vaccinia, typhus fever and Rocky Mountain 

 spotted fever. There are still other diseases of unknown cause about 

 which nothing is known regarding the filterability of the etiological 

 agents of the disease. Scarlet fever, chickenpox and measles belong 



