172 MANUAL OF BACTERIOLOGY. 



may be restrained by one sufficiently fine. The virus of rabies is 

 also probably filterable. 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 sug- 

 gest 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. 



Ashburn and Craig, f in a preliminary report on their re- 

 searches into the etiology of dengue .state that so far as they have 

 gone they have found that the causative agent, whatever its 

 character, resides in the blood of persons suffering from the 

 disease, since intravenous injections of human beings with 

 blood from dengue fever patients produce the disease. The 

 organism is probably ultramicroscopic, since it passes through 

 a Pasteur filter. The incubation period in the persons in- 

 jected is four days and it is the same whether filtered or un- 

 filtered blood is employed. The disease, they state, is not con- 

 tagious, but is conveyed by at least one species of mosquito, 

 Culex faiigans. 



They find on further study J that no organism, either bacter- 

 ium or protozocin, can be demonstrated in the blood. 



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 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. 



*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'Instilul Pasteur. Vol. I., Nos. i and 2. Also Dorset. Invisible 

 Microorganisms. United States Department of Agriculture, Bureau of Animal 

 Industry, Circular No. 57. 1904. 



■\Journ. Am. Med. Assn. V., XLVIII. No. 8., Feb. 23, 1907. p. 693. 



XJourn. Inject. Dis. No. 3, Vol. 4. June 15, 1907. pp. 440-475. 



