150 MANUAL OF BACTERIOLOGY. 



cause may be discovered. The protozoa may play a part in the 

 etiology of some of them. Roux believes that contagious pleuro- 

 pneumonia of cattle is due to a microbe so minute that it is 

 barely visible with the highest powers of the microscope, so that 

 its outlines and its morphology cannot be studied. The virus 

 of this disease remains virulent after being passed through a 

 Pasteur filter, showing that it is small enough to go through 

 its pores. Similar experiments have succeeded with a number 

 of other affections of animals (of which the best known is foot 

 and mouth disease). The virus may pass through a Pasteur 

 or Berkefeld filter of a certain coarseness, but is restrained by 

 one sufficiently fine. The most important of the diseases in 

 this class are rabies and yellow fever. Reed and Carroll found 

 that the infective agent of yellow fever is in the blood, and 

 that the serum could produce yellow fever in a non-immune 

 person after filtration through a Berkefeld filter.* These facts 

 suggest the possibility that failure to find the causes of some 

 other diseases may lie in the fact that their organisms are so 

 small as to be nearly or entirely invisible to the microscope. 



Modes of Introduction. — There are various avenues by 

 which bacteria may enter the body to produce disease. In- 

 fection of the embryo through the ovum or semen seems to be of 

 rare occurrence. Syphilis (which may not be due to bacteria) 

 is transmitted in this manner. The embryo may be infected 

 through the placenta, although not commonly. The bacilli of 

 typhoid fever and the pus-forming bacteria have been known 

 to be conveyed through it. Tuberculosis may also be trans- 

 mitted through the placenta; how frequently is still uncertain. 

 Occasionally the cxanthematous fevers are transmitted from 

 the mother to the fetus. 



*See Reed and Carroll. American Medicine. February 22, 1902. For an 

 admirable review of this subject see Roux. Sur les Microbes dits Invisibles. 

 Bulletin de I'Institut Pasteur. Vol. I., Nos. 1 and 2. Also Dorset. Invisible 

 Microorganisms. United States Department of Agriculture, Bureau of 

 Animal Industry, Circular No. 57. 1904. 



