In biology, a host is an organism that supplies an alien organism called a guest with resources . Depending on the type of host-guest relationship, the host can provide the guest with food , protection , a place to stay and the opportunity to multiply or spread . If the relationship is mutually beneficial, one speaks of symbiosis ; If the host suffers a disadvantage, if it is parasitism , if the guest only uses food waste, there is commensalism . Animals , plants and fungi come into consideration as hosts . The guests include not only microorganisms such as bacteria , but also animals, plants and fungi. The host is usually the larger living being.
Benefit or harm
The host-guest relationship represents a biocenosis in which the two partners can damage each other in different ways, leave each other almost unchanged, or benefit:
- In parasitism , the parasite stays temporarily or permanently or in an alien and usually larger living being (host), at whose expense it lives, for which it has pathogenic properties . The parasite can also be located within the host's cells (so-called host cells ).
- The Commensalism is a biosystem in which a guest ( commensal , without this affecting) benefits from the host material.
- In mutualism , alien organisms live together for mutual benefit, but without mutual dependence.
- The symbiosis is a regular coexistence of alien organisms ( symbiont and host), whereby both partners complement each other physiologically, benefit and depend on each other. This applies, for example, to humans with regard to the intestinal and skin flora , which are part of their microbiome .
Functions of the host
The host can fulfill one or more different functions for the guest:
A reservoir of pathogens can be maintained in it continuously or at least over a long period of time without the host being (significantly) impaired.
In it, the pathogens can develop into stages capable of reproduction. For example, in the case of tapeworms, the larvae in the final host become sexually mature worms; in the case of viruses , new infectious viruses arise from individual non-infectious particles with the help of the final host . In the course of evolution , the pathogen and the ultimate host have adapted to one another in such a way that the guest only damages the ultimate host so little or at least kills it so late that the pathogen can reproduce sufficiently in the ultimate host by then. The fact that the death rate from Marburg fever or Ebola fever in humans is extremely high, for example , indicates that their pathogens are not yet adapted to humans, so that humans are not the ultimate host for these pathogens. Definitive hosts can be further divided into:
- Main host: The pathogen is most often found in or on him.
- Secondary host : A less suitable host than the main host, but which can still be used for reproduction.
- Occasional host: The pathogen can develop normally in him; however, the pathogen prefers other hosts.
- Random host: It is so seldom attacked that it does not play an important role epidemiologically. In contrast to the false host, however, further development is possible in it or from it a further transfer / continuation of the development cycle. For example, humans are a fortuitous host for toxoplasma .
Further development in it is impossible and / or the pathogen cannot be absorbed by a final host from there. Examples in which humans are a false host are the fox tapeworm or the Candiru fish, which is native to the Amazon . A random host can also be a false host. For example, humans are accidental hosts for toxoplasmas and usually also false hosts.
An intermediate host is an organism that takes up early forms of development (e.g. larvae or juvenile stages of a parasite) into its body, enables them to further develop (especially asexual reproduction and / or metamorphosis ) and finally to transfer to another organism. Some parasites or pathogens have several different intermediate hosts.
The transport host is also referred to as a paratenic , batch or collective host . It harbors infectious stages of a parasite, some of which migrate and attack various organs, but cannot multiply or develop further structurally. Such a host only serves as a vehicle for the parasite. The parasite can, for example, adhere to the extremities of the transport host or pass unchanged through the intestinal tract. An external infection of the transport host is sufficient here for an infection to be transmitted ( contact or smear infection ). Examples are house and house flies, blow flies and other insects. Usually this term only applies to arthropods . Although transport hosts are of great importance for the epidemiology of a parasitosis , since they often contribute to the geographical distribution of a parasite, they are not necessary for the maintenance of its life cycle.
Human pathogens pathogens can (for example, from the feces of wild animals derived) in plants commonly roots penetrate stem leaves, shoots and fruits, infect and multiply there. Feeding or sucking stings by insects can also be entry points.
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