From Academic Kids
Parasitism (in Greek: παρασσυτισμός) is an interaction between two organisms, in which one organism (the parasite) benefits and the other (the host) is harmed. Parasitism can be considered a special case of predation since their effects on the host are similarly, though not equivalently, detrimental. Parasites that live inside the body of the host are called endoparasites (e.g., hookworms that live in the host gut) and those that live on the outside are called ectoparasites (e.g., mosquitos). A parasite that kills its host is called a parasitoid. Some parasites are social parasites, taking advantage of interactions between members of a social host species such as ants or termites to their detriment. Kleptoparasitism involves the parasite stealing food that the host has caught or otherwise prepared.
It is important to note that "benefit" and "harm" in the definition of parasitism apply to lineages, not individuals. Thus, if an organism becomes physically stronger as a result of infection but loses reproductive capabilities (as results from some flatworm infections of snails, that organism is harmed in an evolutionary sense and is thus parasitized.
Many endoparasites acquire hosts by passive mechanisms, such as the nematode Ascaris lumbricoides, an endoparasite of the human intestine. A. lumbricoides produces large numbers of eggs which are passed from the host's digestive tract into the external environment, relying on other humans to inadvertently ingest them in places without good sanitation. Ectoparasites, on the other hand, often have elaborate mechanisms and strategies for finding hosts. Some aquatic leeches, for example, locate hosts by sensing movement and then confirm their identity through skin temperature and chemical cues before attaching.
The hosts of parasites often evolve elaborate defensive mechanisms as well. Plants often produce toxins, for example, which deter both parasitic fungi and bacteria as well as herbivores. Vertebrate immune systems can target most parasites through contact with bodily fluids. Many parasites, particularly microorganisms, evolve adaptations to a particular host species; in such specific interactions the two species generally coevolve into a relatively stable relationship that does not kill the host quickly or at all (since this would be detrimental for the parasite as well; but see parasitoid).
Sometimes, the study of parasite taxonomy can elucidate how their hosts are similar or related. For instance, there has been a dispute about whether Phoenicopteriformes (flamingos) are more closely related to Ciconiiformes (storks and related groups) or to Anseriformes (waterfowl and allies). Flamingos share parasites with ducks and geese, so these groups are thought to be more closely related to one another than either is to storks.