BACTERIA IN DISEASE. l6l 



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 mor- 

 phology can not be studied. The virus of this disease re- 

 mains 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 

 Berkenfeld filter of a certain coarseness, but is restrained 

 by one sufficiently fine. The most important of the diseases 

 in this class is 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 Berkenfeld filter. 1 These 

 facts suggest the possibility that failure to find the causes 

 of some other diseases may lie in the fact that their organ- 

 isms 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. 

 Infection 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 com- 

 monly. The bacilli of typhoid fever and the pus-forming 

 bacteria have been known to be conveyed through it. 

 Tuberculosis may also be transmitted through the placenta, 

 how frequently is still- uncertain. The comparatively com- 

 mon occurrence of endocarditis on the right side of the 

 heart in the fetus may be due to placental infection. Oc- 



1 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. I and 2. 



