Obligate parasite

An obligate parasite or holoparasite, is a parasitic organism that cannot complete its life-cycle without exploiting a suitable host. If an obligate parasite cannot obtain a host it will fail to reproduce. This is opposed to a facultative parasite, which can act as a parasite but does not rely on its host to continue its life-cycle. Obligate parasites have evolved a variety of parasitic strategies to exploit their hosts.

It is advantageous for the parasite to preserve the health of their host when this is compatible with their nutritional and reproductive requirements, except when the death of the host is necessary for transmission.

Obligate parasitism is exhibited in a range of organisms, with examples in viruses, bacteria, fungi, plants, and animals. They are unable to complete their development without passing through at least one parasitic stage which is necessary to their life-cycle.

Whether one regards viruses as living organisms or not, they cannot reproduce except by means of resources within living cells, and accordingly, it is convenient and customary to regard them as obligate intracellular parasites.

Among the Vespidae family, Vespula austriaca is an example of an obligate parasite; its common host is Vespula acadica. In the genus Bombus, B. bohemicus is an obligate parasite of B. locurum, B. cryptarum, and B. terrestris.

Parasitic life-cycles involve the exploitation of at least one host. Parasites that infect a single species are said to have direct life-cycles. For example, the hookworm species Necator americanus. Parasites that infect more than one host are said to have a complex or indirect life-cycle. For example, the Malaria plasmodium.

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