Parasite

noun

1. A person who lives on other people's efforts or expense and gives little or nothing back.
2. A sycophant or hanger-on.
3. An organism that lives on or in another organism of a different species, deriving benefit from living on or in that other organism, while not contributing towards that other organism sufficiently to cover the cost to that other organism.

   "Lice, fleas, ticks and mites are widely spread parasites."

4. A climbing plant which is supported by a wall, trellis etc.
5. A retainer or companion of an ancient Celtic warrior, who praised him in song or poetry at gatherings; a bard.