parasitic Definition

parasite
noun

1. A plant or animal that for all or part of its life obtains food and physical protection from a living organism of another species (the host¹ noun 5) which is usually damaged by and never benefits from its presence.

Thesaurus: bloodsucker, leech, sucker, epizoon, endophyte.
2. derog
A person who lives at the expense of others, contributing nothing in return.
Thesaurus: freeloader (US slang), hanger-on, scrounger (slang), sponger (slang).

Derivative: parasitic
adj

Derivative: parasitically
adverb

Etymology: 16c: from Greek parasitos someone who lives at another's expense.