Section I. 



SANITARY SCIENCE AND HYGIENE, 



ADDRESS BY THE PRESIDENT, 



R. GREIG-SMITH, D.Sc, 



Madcay Bacteriologist to the Linnean Society of New South Wales. 



AIR INFECTION. 



In the study of the spread of infectious diseases it is of the greatest 

 importance to know the means by which micro-organisms are conveyed 

 from one place to another, together with the effect of the conditions of 

 environment upon their vitality during transportation. Immense 

 advances have been made during the past few years in our knowledge 

 of the etiology and propagation of many diseases, and especially of 

 the insect-borne diseases of men and animals in tropical countries. 

 These have, for the most part, been traced to various protozoa which 

 spend a portion of their life-cycles in the bodies of certain biting insects 

 such as mosquitoes, biting flies, and ticks. (a) To discuss these advances 

 would absorb more time than I have at my disposal, and, moreover, 

 the subject is so fascinating that it has been reviewed from time to 

 time by many able writers. I will therefore restrict my address to 

 the consideration of the aerial transportation of bacteria from a source 

 of infection to a susceptible individual. 



In considering the conveyance of bacteria in this manner, the first 

 point that we must bear in mind is that these micro-organisms are of 

 extremely small size. They are so minute that a million of ordinary 

 dimensions weigh something like one three-hundred-thousandth part 



(a) In the case of a disease conveyed from man to man by means of a biting 

 insect, we should consider the matter from two points of view — that of the man 

 and that of the insect. The latter aspect of the case is apt to be forgotten. A 

 healthy insect may bite a healthy man and suffer no ill effects ; but should an 

 Anopheles mosquito bite a malarial patient, or a man who has the malarial parasite 

 in his blood, the .insect will become diseased. A diseased insect is, per se, of no 

 importance whatsoever ; but if, after a certain period, it should bite one or more 

 persons the infinitely imimportant becomes converted into the infinitely important, 

 for the bite means the conveyance of the malarial j^arasite, which wiU multiply 

 enormously in the blood and give rise to the symptoms which we know as the disease. 

 Given a district in which there are healthy Anopheles, the coming of even one 

 parasite-containing person may be the lighting of the fuse that fires an explosive 

 epidemic of the disease. Our insects should therefore be guarded from infection. 

 This is carefully attended to in the tropical regions of America, where patients 

 who are suffering from yellow fever are enclosed in niosquito-proof cages 

 in order to protect the " day-mosquito " {Stegomyia fasciata), which is the insect- 

 vehicle of the disease-producing parasite. Thus the transference of the d'sease 



