HOW PARASITIC AND PATHOGENIC BACTERIA REACH MAN 91 
1. Air-borne Infection. Bacteria wliich cause ])r()p-essive disease, 
particularly of the respiratory tract, are discharged from the diseased 
body priucipally through the mouth and nose and find lodgment in 
the environment of the ])atient through the medium of the air, from 
whence they settle upcm various substances, as food, clothing and 
walls and floors of rooms. These bacteria probably do not proliferate 
to any extent outside of the body, but they resist drying and may 
remain fully virulent for considerable periods of time and potentially 
able to infect a certain proportion of those individuals who maj^ be 
exposed to them. 
These air-borne infections are transmitted in at least two rather 
distinct ways: (a) by dust, and (6) by droplet infection. 
(fl) Organisms which are transmissible through dust must first 
of all be able to survive considerable periods of drying. The larger 
particles of dust to which bacteria may become attached soon settle 
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 natur- 
ally stir up particles which have settled from the air, and even larger 
particles may be resuspended in this way. Tuberculosis has fre- 
quently been suspected of having been transmitted through the inha- 
lation 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 contracted 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 empha- 
sized that the transmission of disease through dust is not necessarily 
a very direct one, because the inciting organisms may pass a very 
considerable period of time in dust before they reach a favorable host. 
In this sense, transmission of disease by dust is a relatively latent one. 
{h) Droplet hifedion. — Flugge^ and his pupils were the first to 
demonstrate that minute droplets of spray may be eliminated from 
the mouth during talking, sneezing and coughing, although the expired 
air in ordinary quiet breathing is sterile.^ These droplets are fre- 
quently carried through the air for some distance, even as much as 
10 meters in a quiet room. Thus, Hutchison^ found that B. pro- 
digiosus could be carried 50 meters from a room through a corridor 
and up a flight of stairs. Usually the more minute particles remain 
1 Ztschr. f. Hyg., 1897, 25, 179; 1899, 30, 107. 
2 Straus: Ann. Inst. Pasteur, 1888, 2, 181. 
3 Ztschr. f. Hyg., 1901, 36, 223. 
