animal body fare ill in the competition with hardy sapro- 

 phytic colleagues outside the body, and if certain individuals 

 do survive, these, whilst acquiring the capacity to live upon 

 extra-corporeal nutriment, diminish at the same time in 

 pathogenici ty. In other words, the conditions select a strain 

 tending more and more towards saprophytism and away 

 from parasitism. For these reasons it behoves us to be 

 especially on the look out for any machinery through the 

 agency of which bacteria may be rapidly conveyed from 

 sick to healthy, and their travels made easier for them. 



In not a few diseases the infective agent usually passes 

 almost directly from patient to patient such are syphilis, 

 plague-pneumonia, diphtheria, measles, scarlet fever, small- 

 pox, and numerous parasitic skin diseases. The spread of 

 such highly infectious diseases can be dealt with by 

 isolation and segregation "of contacts, for their range is 

 small. 



Of recent years much attention has been bestowed upon 

 the study of the r61e of insects in the transmission of 

 disease. It is not my intention to deal with these dis- 

 coveries as to the essential part played by insects in the 

 transmission of protozoal parasites made by Smith and 

 Kilbourne, Bruce, Ross ; Reed, Carrol, and Agramonte in 

 the case of the transmission of Texas fever, nagana, sleeping 

 sickness, malaria of birds and animals, and yellow fever 

 respectively. It may, however, be pointed out that it was 

 these researches which focused attention upon the possi- 

 bility of bacterial diseases being conveyed by a similar 

 agency. In the latter case the insect plays a less essential 

 part, and no particular phase in the life-history of the 

 parasite takes place within it. I have accordingly, following 

 a suggestion of my friend Colonel Alcock, 1 employed the 

 term "porters" to describe this passive role of insects in 

 the spread of contagion. 



HOUSE FLIES. 



A deal of attention has been paid to flies lately, in view of 

 their possible influence in the dissemination of infection, 

 more particularly in the case of such diseases as cholera, 

 typhoid fever, and infantile diarrhoea, where the infective 

 agent escapes from the intestine and new infections are taken 

 in by the mouth. 



The first question is, Can the fly convey infection ? Before 

 referring to the" numerous experiments which have afforded 

 an answer, I will briefly refer to those points in the life- 

 history, structure, and habits of the house fly which are of 

 assistance in appreciating how it may play such a role. These 

 subjects have been submitted to careful inquiry during the 



1 Entomology for Medical Officers, London, 1911. 



