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A parasite is an organism that lives on or within another organism (the host) without providing any benefit to the host, and usually causing harm.

Organisms that live on or within another organism without causing harm, and often creating a beneficial relationship are called symbionts.

Parasites often feed on the host, but under normal conditions without causing fatal injury, since this is usually detrimental to both the parasite and the host. The confined spaces of aquariums and ponds however, allow parasites to multiply and re-infect fish several times, and this often causes death to the affected fish.

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