CHANNELS OF INFECTION 15 



influenza (la grippe), of the pulmonic form of bubonic 

 plague, of " wool-sorter's disease " (pulmonary an- 

 thrax), and of pneumonia. In diphtheria the initial 

 infection commonly occurs upon the surface of the 

 tonsils. This is also, no doubt, true in the various 

 forms of tonsillitis and possibly in scarlet fever. In- 

 deed there is good reason to believe that the ton- 

 sils constitute the avenue through which infection 

 occurs, occasionally at least, in a considerable number 

 of diseases of this class, including tuberculosis. The 

 exact knowledge which has been gained during the 

 past twenty years has made it evident that infection 

 through the medium of the air is by no means as 

 common as was formerly believed. We now know 

 that malarial fevers and yellow fever are not con- 

 tracted in this way, but that they result from inocula- 

 tions made by infected mosquitoes. Some disease 

 germs are quickly killed by desiccation and exposure to 

 sunlight. These are not likely to be carried through 

 the air in a living condition, and consequently the 

 diseases produced by them are not propagated in this 

 way. This is true of Asiatic cholera and to a con- 

 siderable extent of typhoid fever, which diseases are 

 recognised as being essentially water-born. How- 

 ever, the bacillus of typhoid fever resists desiccation 

 for some time, and when the surface of the ground 

 becomes contaminated with the discharges of typhoid- 



