Parasites, in the common acceptation of the term, are presumed to be worms of some kind which infest the internal organs of the higher animals. In reality the word has a much more extensive meaning, as it applies literally to all kinds of organisms which live upon other and higher organisms, and it is quite within the limits of possibility that science may 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 constantly being made to the number of microbe affections.
Parasitism, therefore, in its general sense, applies to a much larger number of disorders than are usually 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. Thus 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 sutlers 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.