92 SAPROPHYTISM, PARASITISM, AND PATHOGENISM 



from the air, but smaller particles may remain suspended for some 

 time, depending on the velocity of air currents and the nature, size 

 and shape of the particles. Dusting and sweeping in rooms naturally 

 stir up particles which have settled from the air, and even larger par- 

 ticles may be resuspended in this way. Tuberculosis has frequently 

 been suspected to have been transmitted through the inhalation of 

 infected dust particles, that is, particles of dust which have dried 

 tubercle bacilli adhering to them. Careful investigation has shown that 

 houses in which careless consumptive patients have lived have been 

 responsible for the transmission of tuberculosis. The ward-room of 

 a battle ship is known to have become infected with tubercle bacilli 

 early in its career and at least two successive details of officers con- 

 tracted tuberculosis in this place. Guinea-pigs exposed on the floor 

 of these so-called tuberculous rooms are quite frequently successfully 

 infected with the tubercle bacillus. 



The extent to which dust dissemination is a factor in transmitting 

 disease, however, is not at all definitely known. It must be emphasized 

 that the transmission of disease through dust is not necessarily a 

 very direct one, because the inciting organisms may pass a very con- 

 siderable period of time in dust before they reach a favorable host. 

 In this sense, transmission of disease by dust is a relatively latent one. 



(6) Droplet Infection. Fliigge 1 and his pupils were the first to 

 demonstrate that minute droplets of spray may be eliminated from the 

 mouth during talking, sneezing and coughing. These droplets are 

 frequently carried through the air for some distance, even as much as 

 ten meters in a quiet room. Usually the more minute particles remain 

 suspended in the air for some time. The possibility of droplet infection 

 has been definitely proven in the following manner: Agar plates 

 containing sodium carbonate are placed at various heights and dis- 

 tances from the experimenter, who places in his mouth a solution of 

 phenolphthalein and then talks in a natural manner, expelling droplets 

 containing phenolphthalein during his speech. This dye is transmitted 

 with the droplets until they reach the agar plates, where bright red 

 spots are produced which are very readily observed. In like manner, 

 cultures of B. prodigiosus placed in the mouth will infect agar plates 

 at similar distances. 



The transmission of disease by droplet infection may be, and fre- 

 quently is, a very direct one. Bacteria which are air-borne or borne 

 by droplets may remain alive for several weeks in indirect sunlight, 



1 Ztschr. f. Hyg., 1897, xxv, 179. 



