PHENOMENA OF INFECTION. 87 



to transmit at times other bacterial diseases such as 

 Asiatic cholera, bubonic plague, leprosy, tuberculosis, 

 small-pox, etc. 



But where an insect acts as host for a pathogenic 

 agent, an entirely different condition is presented. Here, 

 while the contamination of the host is accidental in so 

 far as the latter is concerned, it is a necessary sequence 

 on the part of the pathogenic agent. Agents of this 

 class are alternately parasitic upon human beings and 

 insects. Upon each host a supplementary develop- 

 ment begun in one is continued in the other. In no 

 other way can growth from egg to adult (life-cycle) be 

 accomplished. In addition to this remarkable pro- 

 vision, each agent, through the workings of a fixed bio- 

 logical law, is peculiarly restricted to especial hosts 

 without which it cannot develop. It follows, therefore, 

 that the propagation of diseases by insect-hosts is 

 absolutely contingent upon the presence of that partic- 

 ular species of insect which is the agent's natural host. 



Hosts may belong to the class of suctorial (biting) 

 insects, or to such as are likely to get into our food or 

 drink. One instance of a disease due to a protozoan 

 which is transmitted by an insect (mosquito), is fur- 

 nished by malarial fever; another, by sleeping-sickness 

 (trypanosomiasis), of which the African stinging fly 

 glossina is the host. 



Among larger organisms which themselves produce 

 pathological conditions are insects, flukes, worms, etc. 



