METHODS AND CHANNELS OF INFECTION 66 1 



such as sleeping sickness, nagana, dourine, and mal de caderas. It is 

 difficult to cultivate artificially the pathogenic protozoa outside the 

 animal body in pure culture. The Trypanosoma brucei of nagana and 

 the Trypanosoma lewisi of the rat have been cultivated. The 

 Entamceba coll and the Entamceba tetragena of dysentery, the various 

 types of the Plasmodium malaria, and the Treponema pallidum of 

 syphilis have also been cultivated, and it is stated that under certain 

 conditions the Piroplasma bigemimim 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 in most instances cannot be cultivated and 

 cannot be stained by the ordinary bacteriological methods. The 

 presence of ultramicroscopic organisms has been demonstrated in 

 several ways. For example, when the ordinary bacterial culture is 

 run through a fine porcelain filter, the filtrate contains no microorgan- 

 isms and consequently when inoculated into animals is non-infectious, 

 although if soluble toxins be present there may be evidences of an 

 intoxication. When the viruses or the infected body fluids of men or 

 animals suffering from the diseases mentioned below are passed through 

 a fine porcelain filter the filtrate remains infectious, therefore demon- 

 strating that the viruses or microorganisms are filtrable and are prob- 

 ably so small that they cannot be seen. Examples of diseases due to 

 agents belonging to this class are as follows: hog cholera, yellow fever, 

 foot-and-mouth disease, rinderpest, epithelioma contagiosum of fowls, 

 chicken typhus, horse sickness, acute poliomyelitis, etc. There are 

 several infectious diseases of unknown cause, the viruses of which are 

 not filtrable; 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 to this class. These diseases can be inoculated into 

 animals only with great difficulty and the virus cannot be cultivated 

 or secured in sufficient quantities from the experimental animals for 

 study. A possible explanation of some of these diseases of unknown 

 cause may be found in the proposition that two microorganisms may 

 each produce non-toxic substances, and that when these non-toxic 

 substances come together, a toxic substance may be produced. This 



