486 PATHOGENIC MICROORGANISMS 



Against the assumption of the etiological importance of the influ- 

 enza bacillus, are the occasional failures to find these organisms in 

 early cases, the frequent presence of the bacilli in the throats of 

 normal individuals suffering from no typical influenza, and their 

 presence in interepidemic periods in many pathological conditions 

 observed in clinical influenza ; their frequent presence as complicating 

 invaders in whooping cough, measles, etc., and the antigenic multi- 

 plicity of strains isolated at times of epidemic. The absence of the 

 organisms in most early blood cultures, also, should be added to the 

 negative evidence. 



Investigations of antibodies in the serum of cases that are sick 

 permit of no etiologic conclusions, since secondary invasion might lead 

 to antibody formation, whatever the primary cause. 



Protection experiments with vaccines have been absolutely incon- 

 clusive. 



All of this evidence must be considered in connection with recent 

 experiments upon the possibility of causation by a filtrable virus in 

 influenza, a problem upon which much work has been done, and which 

 we must, therefore, include in our summary as follows : 



FILTRABLE VIRUS AND INFLUENZA. The idea that a filtrable virus 

 might be the etiological agent in influenza is suggested by the char- 

 acteristics of the pure uncomplicated mild cases described above, the 

 extreme infectiousness of the disease, and the negative character or, 

 at least, the lack of uniformity of the bacteriological findings in early 

 cases. In consequence, a considerable number of workers have attacked 

 the problem from this point of view. In 1918, Nicolle and Lebailly 2 

 reported on studies made on the filtration of influenzal virus and its 

 inoculation into animals and man. They filtered the blood and nasal 

 secretions of uncomplicated grippe cases, and instilled them into 

 conjunctival sacs and nasal cavities of several monkeys (Macaccus 

 Sihicus) and into a number of human volunteers. They obtained 

 symptoms within about six days in several monkeys, and in two men, 

 but were unable to carry the infection into a second generation. They 

 concluded that the filtrates of secretions in influenza are virulent, and 

 can infect human beings and certain monkeys by nasal and con- 

 junctival inoculation. 



Very soon after this, Dujarric de la Riviere 3 filtered blood from 



2 Nicolle and Lebailly, Ann. de 1'Inst. Past., 33, 1919, 385. 



3 Dujarric de la Eiviere, Gompt. Rend. Acad. des Sciences, 167, 1918, 406. 



