An organism that derives nutrients from the living tissues of another organism (its host). 
<biology> An organism which cannot survive or reproduce unless it is on another organism. For example: fleas. ... (19 Jan 1998) ... 
an organism that can live and multiply only on another living organism.
Obligate parasite i.e. an organism which is entirely dependent upon another living organism as a source of nutrients.
biotroph, biotrophic, biotrophy 1. A reference to a parasite deriving nutrients from the tissues of a living host. 2. An organism that can live and multiply only on another living organism; such as, parasitic or symbiotic bacteria and fungi, carnivores, herbivores, etc.