Any living organisms are in continuous interaction with each other and with the habitat. Negative relationship between individuals of different types is called an antibiosis. It can be shown in different forms, one of which is the parasitism.
Cohabitation form at which some organisms use other types as the constant power supply or as the habitat, the parasitism is called. He is several types: temporary, constant and nested. The relations at which the parasite uses the owner's organism only for the period of food are called temporary. Such interaction with the environment at bugs, mosquitoes, fleas and others. They are capable to cause significant damage to an individual of other look and even to cause her death (during simultaneous attack of a large number of blood-sicking insects).
The constant parasitism is use by parasites of the victim as food or the habitat a long time. At this form of relationship the death of the owner leads to destruction and an alien organism. Many protozoa (a malarial plasmodium, etc.), roundworms (a trikhina, an ascarid, etc.), flat worms (flukes, tapeworms), arthropods belong to this category (scabby zuden, louses, etc.).
Constant parasites though do not cause fast death, but all the same cause damage to an organism in which settle. Mechanical damages apply suckers, hooks and other devices for deduction on surfaces or in the owner's body. There is a fabric defeat (partial or total destruction) and, therefore, its functions are broken. Not the smaller danger to the infected organism is constituted by the waste products of parasites emitted by them poisons. So toxin of trypanosomes (the carrier a tsetse fly) causes fever in the person and leads to a lethal outcome. There is one more known form of negative cohabitation – nested parasitism. When one organism lays in a nest to other look the eggs. And already foundlings are raised by owners of a nest as the posterity. A striking example of nested parasitism is the cuckoo. The parasitic relations are known also at plants: the dodder, is twisted around a stalk and takes root into it suckers; a raffleziya, the mistletoe and many others eats only at the expense of juice of a plant on which grows. For fight against various parasites it is important to study their life cycle, owners, ways of infection, etc. Today bigger significance is attached to a biological method of fight against parasites that allows to reduce use of poisonous chemicals.