MEDIUMS OF CONVEYANCE. 49 



of conveyance, and where such contact can be re- 

 alized with difficulty communication from person 

 to person rarely occurs (e. g tetanus and hydropho- 

 bia, in which wound inoculation is required). 



It is probable that the air, in the absence Air as 

 of atmospheric currents, would soon become ster- S 

 ile, by virtue of the effect of gravity in carrying 

 the microbes to the earth, and the germicidal ac- 

 tion of sunlight and diffuse light. Of course, these 

 conditions do not prevail, or are not effective, and 

 the air stands as one of the important agencies by 

 which virulent micro-organisms reach the individ- 

 ual, either from other individuals and animals or 

 from inanimate nature. 



For a long time it was supposed that convey- 

 ance through the air takes place chiefly or entirely 

 through the medium of fine particles of dust which 

 are laden with micro-organisms ("dust infection") . 

 It was only through fundamental work by Mugge 

 (1897) and others that it was shown how minute 

 droplets of saliva or mucus from the lungs or nasal 

 passages are fully as important as dust, and, per- 

 haps more important, for the transfer of infection 

 from one individual to another. This is "droplet 

 infection." 



The fields occupied by dust infection and drop- "Dust 



,,... T ^ i ,-, Infection. 



let infection do not coincide exactly. Only those 

 diseases can be concerned in droplet infection in 

 which infected droplets are discharged from the 

 body- i. e., chiefly from the upper respiratory pas- 

 sages. Since the infective droplets of saliva, 

 serum, mucus and pus may become desiccated and 

 pulverized, the field of dust infection, theoretically, 

 includes not only those diseases, but also many 

 others. Thus, dust infected with the pyogenic or- 



