Dictionary of Environmental Health
An organism that derives nourishment (energy) directly from another organism (the host). The term can be taken very widely and there are many different types of parasitic relationship. In general (although not exclusively) the term is commonly applied to organisms that live directly off the host’s body, tissues, fluids or gut contents without killing the host. That some parasites do kill the host is true, but such is likely to be ‘accidental’, as there is usually no longterm benefit in doing so since the parasite loses its living food supplier.
There are many ways of classifying parasites. ECTOPARASITES live on the external body of the host, whilst ENDOPARASITES live within the body of the host. Parasites may be facultative (i.e. are not compelled to survive solely as parasites) or obligatory (i.e.
they cannot live otherwise). Some parasites are permanently so, while others may be temporary or intermittent, using parasitism only as a transitory part of their life history or as an occasional means of obtaining nutrients.
The parasite’s relationship with the host may not always noticeably affect the latter in any way. However, many parasites can cause disease (i.e. they are PATHOGENIC) directly in the host (e.g. Plasmodium spp. cause malaria) and other parasites may act as transmitters of pathogenic organisms from host to host (e.g. the tsetse fly may transmit the organism responsible for trypanosomiasis).
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