1922] Microscopic Parasites and their Carriers 503 



WEEKLY EVENING MEETING, 

 Friday, March 3, 1022. 



Sir James Crichton-Browne, M.D. LL.D. F.R.S., 

 Treasurer and Yice-President, in the Chair. 



C. Morley Wenyon, C.M.G. C.B.E. M.B., 



Wellcome Bureau of Scientific Research. 



Microscopic Parasites and their Carriers. 



Microscopic parasites, like the living things which we see around 

 us, belong either to the vegetable or animal kingdom, and I propose 

 to deal with the latter group alone, and more especially with refer- 

 ence to the various methods by which they are handed on from one 

 host to another, " host " being the term applied to the larger animal 

 or man in which they live as guests, however unwelcome they may 

 be. I wish particularly to draw attention to the relationship existiug 

 between the host and its parasite, and to show how a parasite may 

 avail itself — if one may suppose for convenience that so lowly an 

 organism can deliberately avail itself of anything — of the habits and 

 peculiarities of a blood-sucking invertebrate for the purpose of 

 transferring itself to a second host in order to keep itself in being. 



The one essential characteristic of all living things is the capacity 

 they possess of propagation and maintenance of the species. This 

 characteristic is necessary for their continued existence, and should it 

 fail the species will at once become extinct. The various methods 

 by which parasites in general maintain themselves — methods which 

 are sometimes extremely complicated and necessitate the passage from 

 one kind of host to another— form a most fascinating subject for 

 study which not only is of interest in itself, but which has a direct 

 bearing on the welfare of mankind in the prevention of disease. 



It is a general notion that a parasite is distinctly harmful to its 

 ho^t, and in a wide sense this conception is true But it is easy to 

 realise that if a parasite gains entrance to a host and multiplies so 

 rapidly that the host is quickly killed, the chances of the parasite 

 finding a home in another host may be very small indeed. This 

 danger of extinction is a very real one, for the parasite has a free- 

 living ancestry which led the usual competitive existence amongst 

 innumerable creatures around it. In adopting a parasitic mode of 

 existence it has lost most of the qualities which enabled it to hold 

 its own in the world of living things. When it has adapted itself to 



