Parasitism is a relationship between two or more species, in which a parasite temporarily or permanently exploits the energy of a host. A parasite lives on the outer surface of a host, or inside its body: digestive organs, respiratory organs, venous system, and other organs or tissues.
Frequently a host provides a parasite not only with food, but also with enzymes and oxygen, and offers favourable temperature conditions. In principle, a parasite can meet its living requirements exclusively from the connection with a host. But a host is certainly not inactive against a parasite, and it hinders the development and population growth of parasites with different defensive mechanisms. Therefore, the consequences of parasitism depend on the physiological state of a host and on the external ecological factors which affect it. For this reason we think that parasitism is a phenomenon of the ecological system. From the standpoint of ecological system even parasitism has a positive meaning, for it is a mode of regulation of populations; and it is thus not surprising that among 40,000 animal species living in Central Europe we find as much as 10,000 parasite ones (ratio 1:4).|
External parasites, or ectoparasites, operate on external body surfaces, and internal parasites, or endoparasites, live in body cavities, tissues, and blood of their hosts. Internal parasites have their origins in free-living ancestors which invaded internal parts of hosts through their mouths or anuses (amoebas and worms), or actively penetrated through their skin. Their way into hosts could also be assisted by blood-sucking animals. And this is also the explanation of their evolution.
There is "tension" between a host and its parasite, since the host endeavours to get rid of the foreign body, while the parasite employs new ways to maintain the connection with the host. The host hampers the parasite both mechanically and chemically. Cleaning of skin, peristaltic contraction of digestive apparatus, development of antibodies - these are some modes of defense. Parasites respond to this defense by anchoring themselves with hooks and suckers onto skin, or digestive mucous membrane, and by developing protective devices and substances which lessen defensive capacities of their host.
Parasites moving from one host to another encounter ecological obstacles. When moving, they frequently have to spend a certain period of time on the ground, or in the water, and during this time they are exposed to the effects of abiotic factors.
Russian parasitologist Pavlovski said (in 1939) that the spatial connection of parasites with a particular ecological type of landscape represents a natural focal point.