president's address — SECTION I. 239 



of a grain. It will, therefore, be easily understood that even the 

 faintest breath of wind or the slightest current of air will convey them 

 from one place to another. They undoubtedly possess weight, and 

 compared with other substances they are of much the same specific 

 gravity (3) as particles of coal or boxwood. For this reason they will 

 settle in a perfectly still atmosphere, but such a condition is exceedingly 

 exceptional. The friction and convectional motion of the air will keep 

 them suspended for long intervals. Furthermore, the smaller the 

 microbe the longer will it float. Some bacteria are so small that they 

 readily pass through the pores of our finest porcelain filters, and on 

 account of their extreme size, which is less than that of a ray of light, 

 they have not hitherto been seen with our most powerful ordinary 

 microscopes. Such are the invisible or ultramicroscopical bacteria, 

 examples of which we have in the micro-organisms of South African 

 horse sickness, of pleuro-pneumonia, and of smallpox. (6) Such microbes 

 will probably float about in the air until they perish. 



Patients suffering from infectious diseases are capable of giving 

 off virulent bacteria in several ways. They may be avoided along with 

 the waste products from the intestine and kidneys, or be given off with 

 desquamated epithelium. The excreta are but too frequently thrown 

 upon the ground or lightly covered with earth, with the result that 

 the soil becomes infective. But even before this occurs much danger 

 to the community may arise, as anyone can see who notes the per- 

 sistency with which meat flies and other flies haunt the so-called earth- 

 closets and frequent the rooms in our country townships and our 

 mountain health resorts, whither convalescents resort to recuperate. 



from patient to insect is prevented, and the possibility of the subsequent infection 

 of many healthy persons avoided. The point to be emphasised is that one in- 

 dividual who has the protozoon of a certain disease in his blood may be the means 

 of infecting a number of mosquitoes. In the body of the mosquito the parasite 

 continues and completes an interrupted life cycle. The sporozoites — the terminal 

 products of the cycle — pass with the saliva of the mosquito into the unlucky wight 

 whom it chances to feed upon. Just as the potato is the more vigorous after having 

 been raised from seed, so it is probable that the parasite is more virulent — that is, 

 it can withstand more adverse cu-cumstances or conditions, such as immunity con- 

 ditions of the individual — after having completed its full life cycle in the tissues 

 of its particular insect host. 



Tlie protozoal diseases are in this respect different from the bacterial. Bac- 

 teria have no life cycle to complete in the insect, and they die within their bodies 

 in course of time — sometimes a few hours, sometimes a few days. It is true that 

 a bacterial disease may result from the bite of an insect ; but in such cases the 

 evidence goes to show that the bacteria have either been adhering to the mouth 

 parts or have been voided with the excreta in the vicinity of the bite and have been 

 scratched into the skin by the individual. 



In Australia we have Anopheles anmdipes (\), the carrier of malaria, and 

 Culcx fatigans, the intermediate host of filariasis (2), both of which, upon the 

 advent of parasite-containing individuals, will be the means of transferring these 

 diseases to many other persons. 



[h) The work of Proscher (4), which bears the impress of probability, shows 

 that the micro-organism of smallpox is retained by the porcelain filter, but is in- 

 visible. This may be due to an exceptional neutrophile condition of the cells. 

 He obtained evidence of growth in the alteration of the surface of a special medium. 

 According to other authors, it passes through the Berkefeld filter. 



