DROPLET INFECTIONS. 53 



ing some of the contagious exanthemata, die more 

 quickly. 



The conditions for droplet infection, then, are, conditions. 

 in the main, as follows: First f the micro-organ- 

 isms must be discharged into the air, in viable 

 and virulent condition, and in sufficient quantity, 

 from the respiratory passages of the patient. Sec- 

 ond, they must be able to retain life and virulence 

 for a greater or less period of time, after 

 being liberated in this way. Third, they must be 

 able to use some portion of the respiratory tract, 

 or the gastro-intestinal tract indirectly from the 

 respiratory tract, as an infection atrium. Fourth, 

 the advent of a susceptible person within the zone 

 of infected atmosphere which surrounds the pa- 

 tient during the period of viability and virulence 

 of the excreted organisms. 



In general the same principles also apply to dust 

 infection so far as it concerns the ordinary "air 

 borne" diseases. 



The question is commonly raised as to which Relative 



. ,, . i i -i j Importance. 



is the more important, or more prevalent, dust in- 

 fection or droplet infection. Although it is the 

 tendency at present to assign a minor role to dust 

 infection, it unquestionably is of importance in 

 certain diseases, particularly in tuberculosis. The 

 situation may be conceived to be as follows: In 

 relation to diseases caused by micro-organisms 

 which have little resistance to desiccation and 

 light (e.g.., plague, influenza), droplet infection, 

 implying proximity to the patient, is more likely 

 to occur than dust infection (as in occupying a 

 room vacated by a patient a longer or shorter pe- 

 riod previously). Concerning those caused by or- 

 ganisms which have greater resistance to desicca- 



