52 INFECTION AND IMMUNITY. 



as a consequence of "dusting." Naturally the 

 longer the suspension within the period of via- 

 bility of the organisms the greater the chance of 

 conveyance. 

 Droplet Droplets of saliva, etc.. are discharged into the 



Infection. .1 i -, i i , -, 



air through coughing, sneezing, laughter, and even 

 forcible speaking. This does not occur with ordi- 

 nary respiration. In coughing droplets may be 

 carried as far as thirty feet from the individual, 

 and they may be carried much farther by extrane- 

 ous currents of air. Such droplets may remain 

 suspended in the air for approximately an hour, 

 but the period of their suspension depends on their 

 size, the weight of the organisms they carry and 

 the degree of moisture in the air. In a drier air 

 they become desiccated more quickly and after des- 

 iccation fall more rapidly. 



Laschtschenko, and also Heymann, investigated 

 the dissemination of tubercle bacilli by coughing 

 in pulmonary tuberculosis. In 40 per cent, of the 

 cases guinea-pigs which, were placed even at con- 

 siderable distances from the patients contracted 

 tuberculosis of the lungs or bronchial lymph 

 glands. The bacilli have been demonstrated mi- 

 croscopically in such droplets by numerous ob- 

 servers. Leprosy bacilli are carried similarlv 

 (Schaifer). 



On account of their content in mucin the drop- 

 lets adhere closely to solid surfaces, where they 

 soon dry and become harmless unless they are 

 dislodged by some violence. 



When contained in such dry droplets the tu- 

 bercle bacillus lives for about three days in the 

 light and eighteen days in the dark. Most other 

 organisms, except the spore-bearing and those cans- 



