n. an intimate and obligatory association between two different species of organism (symbionts) in which there is mutual aid and benefit. Compare commensal; mutualism; parasite.
n. an organism that lives in close association with another of a different species without either harming or benefiting it. For example, some microorganisms living in the gut obtain both food and a suitable habitat but neither harm nor benefit humans. Compare symbiosis. —commensalism n.... commensal
n. any living thing that lives in (see endoparasite) or on (see ectoparasite) another living organism (see host). The parasite, which may spend all or only part of its existence with the host, obtains food and/or shelter from the host and contributes nothing to its welfare. Some parasites cause irritation and interfere with bodily functions; others destroy host tissues and release toxins into the body, thus injuring health and causing disease. Human parasites include fungi, bacteria, viruses, protozoa, and worms. See also commensal; symbiosis. —parasitic adj.... parasite
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