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Parasite is an organism that lives on the food ingested by
its host. Parasites lodging on the exterior of their host are
ectoparasites and those lodging in the internal organs or
tissues of the host are endoparasites. Obligate parasites
survive and reproduce only in or on a living host.
Parasites usually adopt a very specialized way of life,
involving one or several hosts; those on the human body
include lice, various bacteria, protozoa and fungi, which
cause many characteristic diseases. Facultative parasite
is an organism which can live entirely on organic matter
from the dead or decaying tissue of plants or animals (as
saprophyte) but can also live as a parasite on a live host.
Destruction of the host reproductive organs by
parasites is known as parasitic castration. The action of
smut fungi in the grain of wheat or the conversion of
stamens and carpels into petals in plants infested by
nematodes are instances of parasitic castration.