METHODS AND CHANNELS OF INFECTION 665 



dust and carried by currents of air. This, however, has been shown 

 not always to be the case and we now know that infectious micro- 

 organisms may be carried on small particles or droplets of sputum or 

 moisture. These two types of aerial infection are known, respectively, 

 as dust and droplet infection. 



Dust Infection. Infectious microorganisms to remain virulent and 

 be able to produce infection must be able to successfully resist drying 

 after being affixed to particles of dust. After being dried the particles 

 are frequently moved and whirled about by air currents. The larger 

 particles of material quickly settle down but the small, almost invisible 

 pieces of dried material may remain suspended for three or four hours. 

 It is these small particles which are usually inhaled or deposited on the 

 skin and mucous membranes of normal individuals that produce in- 

 fections providing the microorganisms have not been killed by drying 

 or exposure to sunlight. Bact. tuberculosis is sometimes carried in this 

 way as well as certain other pathogens. The fact that small-pox virus 

 remains active after drying indicates, at least, that dust containing 

 it. may be infectious. The extent of such dissemination is quite limited. 



Droplet Infections. It has been demonstrated that during the proc- 

 esses of talking, coughing, and sneezing, small bubbles or droplets of 

 sputum are thrown out into the air. These particles remain suspended 

 for some time and may be inhaled or deposited elsewhere. It is sur- 

 prising the distance that these small particles may be carried. It is 

 stated that they are frequently thrown out thirty feet or more. It has 

 been shown that Bact. tuberculosis is rarely thrown out over four or five 

 feet by the cough of the tuberculous individual. It should be re- 

 membered that these bacteria will remain alive two to three weeks 

 when in the dark but that they live only a few hours when exposed 

 to the sunlight. The pathogenic microorganisms or viruses which 

 are commonly disseminated by droplets of moisture are those of 

 whooping cough, mumps, measles, influenza, epidemic meningitis, 

 pneumonia, and pneumonic plague. 



Air-borne infections rarely occur, as previously stated, and are not of 

 great importance in the open air where sunlight has free access to the 

 disease germs but this type of infection sometimes occurs in crowded 

 quarters such as dark shops, schools, tenements and railway trains. 

 However, the factor of direct contact must be given its due weight in 

 such instances. 



