THE president's ADDRESS. 



bef^in their cycle of development in the host by very active 

 endoo-enous multiplication, overrunning some particular organ or 

 tissue, and producing a sick and enfeebled condition which brings 

 the host often to the verge of death, and sometimes even kills it. 

 As a rule, however, animals attacked by coccidiosis, as the disease 

 is termed, recover from the malady owing to the fact that when 

 the host is brought very low, its condition appears to react on 

 the parasite, with the result that the parasite ceases to multiply 

 endogenously and enters upon a different reproductive phase ; 

 it produces resistant stages destined to infest new hosts, in the 

 form of cysts and spores, which are inactive and do not absorb 

 nutriment, but pass out of the body of the host. In consequence 

 the diseased animal loses its parasites, or the greater number of 

 them, and the tissues are left ia peace to regenerate and repair 

 their injuries, provided they have not gone beyond the point at 

 which recuperation is possible. 



If the action and reaction of host and parasite were relations 

 dependent simply on the number or bulk of parasites present in 

 the body of a given host, the problems of parasitism would be 

 comparatively simple. Examples can be brouglit forward to 

 show that in many cases the effects produced by Protozoan 

 parasites cannot be explained, either by the number of parasites 

 present or by the aggregate bulk of the parasites in proportion to 

 that of the body of the host. The effect produced by a given 

 species of parasite on a given species of host is a specific reaction, 

 which differs markedly when one of the two factors is varied. 

 It is not uncommon to find insects with their digestive tract or 

 body cavity crammed with parasitic gregarines, of relatively 

 large size, but causing no apparent inconvenience to the host. 

 On the other hand large mammals may be killed by minute 

 organisms in scanty numbers. A better comparison is furnished 

 by considering closely allied parasites and hosts respectively. A 

 rat may have its blood swarming with TrypanosoTria levnsi, without 

 being apparently any the worse for it. On the other hand, in 

 a man dying of sleeping sickness, caused by T. gamhiense, or in 

 a ruminant dying of nagana (tsetse-fly disease), caused by 

 T. brucii, the trypanosomes may be so scanty as to be exceedingly 



