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A parasite is an organism which engages in a non-mutualistic relationship with another organism, by living either on or within this organism, and experiences an increase in fitness at the expense of the host organism. Parasites can reduce the fitness of their hosts in a multitude of ways, such as by impairing their secondary sex characteristics.

In order to function, in terms of growth and reproduction, a parasite is completely dependent on its host. It is important to mention here that a parasitoid is not the same thing as a parasite. Rather, a parasitoid is a different type of parasite which eventually kills its host once developed. Conversely, parasites rarely kill their hosts.

Parasites come in many forms including bacteria, viruses and protozoa for example. The term parasite generally refers to organisms such as bacteria, which are able to be transmitted throughout the same species. These are known as microparasites.

On the other hand, there are also macroparasites, which are slightly larger in size in comparison to microparasites. These include protozoa for example. Macroparasites are parasites which require multiple host organisms throughout their lifetime as they experience different life stages.

Furthermore, within the parasite category there are multiple different classifications which are dependent upon the particular life cycle of a parasite and its subsequent host interactions. For example, endoparasites are organisms which live within their host’s cells, with intercellular parasites taking up cells within the host organism’s body and intracellular parasites invading cells on the outside of the host’s body.

Parasites are interesting organisms which are not representative of a specific type of organism, but rather a specific lifestyle. Parasites are an important part of all ecosystems and the feeding relationships which take place between species. Therefore, understanding their complexity is important. 



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