240 president's address — section i. 



Infective soil is a very dangerous substance in Australia, for it is 

 so easily blown by our high winds upon the roofs of cottages, from 

 which it is washed by the rain into the taliks, which most of our 

 country inhabitants depend upon for their supply of water for do- 

 mestic use. 



It is extraordinary how long some infectious microbes remain in 

 the bodies of patients after convalescence and return to health. For 

 example, convalescents from typhoid fever carry about with them and 

 distribute for months, and even for years, the germs of the malady from 

 which they have recovered. (5) The typhoid bacteria, perhaps in 

 prodigious numbers, leave the body in the urine. Such convalescents 

 have been known to start epidemics in localities where the inhabitants 

 have been unwittingly using a polluted water. (6) Typhoid is one of 

 the cases in which the patient should be taught to understand the danger 

 which he may be to the community. He should be under observation 

 for some time after convalescence, and should never be employed in 

 certain industries — such as dairying — until it has been proved by 

 bacteriological examination that he is no longer capable of giving off 

 virulent bacteria. (7) 



But the most dangerous of all methods of aerial infection is the 

 dissemination of microbes with the exhalations from the mouth and 

 nose of infectious individuals, because these emanations are invisible, 

 and the public is not inclined to view them with the same horror that 

 it has for the infectious material discussed in the last, two paragraphs. 

 These invisible exhalations are the more dangerous because we can 

 boil our water and cook our food, but we do not filter or sterilise the 

 air that we breathe. As it is by this means that phthisis and many 

 diseases of lesser importance are conveyed from man to man, and 

 as the subject is of much importance, I propose to deal with it at 

 some length. 



In ordinary gentle breathing, the air given off by way of the 

 mouth and nose is nearly sterile ; only those bacteria that entered 

 and have not been caught by the mucous membranes of the respiratory 

 tract are present. Even when the saliva and moisture of the passages 

 are teeming with bacteria there is no opportunity given to them to 

 become detached under ordinary circumstances. The case, however, 

 is quite different when air rushes rapidly and suddenly through the 

 channels of communication between the lungs and the atmosphere, as 

 in the acts of coughing, sneezing, or even of loud and energetic speak- 

 ing. Under these circumstances the exhaled air carries with it minute 

 microbe-holding particles or bubbles of spray of about one two-thousand- 

 five-hundredth of an inch (1-lOOmm.) diameter.(8) With the healthy 

 individual not exposed to infection the microbes are, to all intents and 

 purposes, harmless. But when the individual is infected with certain 

 diseases — such as pulmonary consumption, whooping-cough, diphtheria, 

 influenza, and even other diseases like typhoid, as authenticated by 

 Jehle (9) — the spray may be deadly. 



In proving the emissibility of bacteria in this way, experiments 

 have been made with saHvary bacteria, with pathogenic microbes 



