The Epidemiology of Parasitic Diseases, With Special Reference to Studies With Nematode Parasites of Sheep.
"A parasite’s existence is usually an elaborate compromise between extracting sufficient nourishment to maintain and propagate itself, and not impairing too much the vitality or reducing the numbers of its host, which is providing it with a home and a free ride." In these words Elton, the ecologist, directs attention to the manifold relationships between host and parasite, but essentially with reference to relationships after the parasite has established itself in the host. The parasitic stage is but half the life-history, so far as most of the strongyloid nematode parasites of sheep are concerned. The free-living stages in faeces and on the ground are of particular significance when control measures have to be formulated. The parasite must therefore adjust its way of living to at least two environmental conditions, namely, that provided by the host and that prevailing in its non-parasitic habitat. In the more complex life-cycles in which intermediate hosts are concerned, further environmental hazards present themselves.
Under "natural" conditions there is a nicety of balance between host and parasite......" http://www.readcube.com/articles/10.1111%2Fj.1751-0813.1948.tb01569.x?r3_referer=wol&tracking_action=preview_click&show_checkout=1
Type/Antitype - anyone?