parasitism is the form of mutual relationship between two organisms which occurs in such a way that one organism always get benefit and the other always gets harm. the one getting harm may or may not be killed due to it.
there are many common parasitic relationships we see in eco system.for example, when a tick sucks blood out of a dog. it is causing harm to dog by decreasing the content of blood however getting benefit for itself since blood is its food and the nutrients present in blood are source of energy for tick.
similarly, malaria is caused in human by a parasite called plasmodium. it enters into the blood and causes a disease called malaria which can even lead to the death of affected individual,the parasite itself stays healthy and gets benefit.
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