Parasitism: biological interactions, types, and impacts

Parasite is etymologically derived from ancient Greek παράσιτος parásitos, German 'eating from another, parasite', which goes back to ancient Greek παρά pará, German 'next to' as well as ancient Greek σῖτος sītos, German 'grain, food [made from grain]'. Hereby was originally meant the taster at sacrificial feasts, who thereby came to be fed without performance. From there the meaning passed to the lickspittles of ancient comedy, who sought to procure free meals by fine words. A switch to a biological meaning in the sense of a living thing that lives in or on others (see ectoparasites and endoparasites) and extracts nutrients from them occurred in the 18th century. From there, the meaning soon returned to the social field, for example in Enlightenment polemics against the nobility or in the anti-Semitic stereotype of the Jewish parasite.

The German word Schmarotzer for a parasite comes from the Middle High German smorotzer, meaning beggar.

Parasites are highly specialized organisms. Their habitat is usually restricted to a few host species, and it is not uncommon to find only one host species. Parasitism manifests itself in very diverse forms. There are doubtful cases where parasitism is difficult to distinguish from other interactions between species. Parasitism is by no means a rare phenomenon, as the vast majority of all living things parasitize. With the caveat that exact numbers cannot be determined, the ratio is thought to be as high as 4:1. The appearance of the next generation in the host is called sponsorship.

In general, a parasite is highly dependent on its host. Parasitism can be related to various host factors such as body substance, food supply, oxygen demand, osmotic, pH conditions or heat balance.

Parasitism is ubiquitous, so virtually all living things have to deal with it. It is not uncommon to find dozens of different parasites on or in a single creature, not counting microorganisms. In wood mice, no fewer than 47 parasitic species were found.

Depending on the extent of the parasite infestation, the burden on the host varies. Even if parasite infestation does not cause life-threatening damage to the host, it always has a negative effect on its growth, well-being, susceptibility to infection, reproduction or lifespan. Thus, toxic metabolic products of the parasite, internal or external injuries left behind or deprivation of food can result in a shortening of life, especially in the case of further unfavourable environmental conditions. Hosts, however, are by no means passive towards their parasites, but are usually able to limit the number and damaging effect by suitable defence mechanisms. In a joint evolution (co-evolution), hosts and their parasites adapted to each other. Thus, at each stage of evolution, a balance developed in which the parasite benefits without harming the host, which is its "basis of existence", more than necessary or even destroying it completely (the same mechanism exists in infectious diseases between pathogen and host with regard to virulence, disease progression and immune defence).

Many parasites parasitize in different hosts during their development. A distinction is made between intermediate hosts and the final host. Sexual reproduction usually only takes place in the final host.

Organisms that are attacked without the continuation of the parasite's development cycle being possible are referred to as the false host. Often, the parasite is poorly adapted to its maladaptive host, so that the maladaptive host is more damaged by the parasite than the host.