160 HEALTH AND DISEASE 



yet be able to demonstrate that every form of disease depends on the 

 existence of a parasite. 



At the present time it is known that certain diseases — for example, 

 glanders, tuberculosis, and numerous others — are due to the presence of 

 minute organisms only to be recognized under the higher powers of the 

 microscope. A considerable number of diseases occupy a doubtful position 

 in this respect, and a much larger proportion have not yet been suspected 

 to be consequent on the presence of microbes, but additions are con- 

 stantly being made to the number of microbe affections. 



Parasitism, therefore, in its general sense, applies to a much larger 

 number of disorders than are u.sually included in that definition. This 

 section, however, will be exclusively concerned with those diseases which 

 are connected with the presence of parasites which are not microbes. 



It will be an advantage in the beginning to dispose of certain terms 

 which will of necessity be used frequently in the following pages. 



Parasites belong both to the animal and vegetable kingdoms, and they 

 infest higher organisms of both kingdoms interchangeably, i.e. a parasitic 

 plant may invade an animal as a parasitic animal may invade a plant, and 

 the " host ", as the invaded animal or plant is called, may harbour both 

 kinds at the same time. 



Parasites form only one division of the lower organisms which infest 

 higher organisms. The general term which indicates the condition of 

 which parasitism is a part is Symbiosis, meaning merely association. 

 Tlius some organisms live upon higher organisms to the advantage of 

 both, and this form of symbiosis is described as mutualism. In another 

 division the lower organism attaches itself to a higher one for the purpose 

 of feeding on the remains of the food which the higher animal scatters 

 around; this kind of association is distinguished as commensalism. It 

 is obvious that the higher organism gains no benefit from this connection, 

 but at the same time it suffers no harm. 



In a third example of symbiosis, or the association of lower with 

 higher organisms, the latter is injured more or less by the invasion of 

 the former, which in the exercise of its functions robs its host of a certain 

 amount of nutriment, and during the process of appropriation frequently 

 excretes poisonous substances which are destructive to the tissues of the 

 infested animal or plant. This is true Parasitism. 



Certain terms are used to define the position of the parasites in or on 

 the body of the host. For example, the terms ecto-parasites or epi-para- 

 sites include all the organisms which locate themselves on the surface; 

 ento- or endo-parasites include all organisms which invade the interior of 

 the body. 



