From Merriam-Webster’s Online Dictionary

Main Entry: par·a·site 
Pronunciation: \ˈper-ə-ˌsīt, ˈpa-rə-\
Function: noun
Etymology: Middle French, from Latin parasitus, from Greek parasitos, from para- + sitos grain, food
Date: 1539
1 : a person who exploits the hospitality of the rich and earns welcome by flattery 2 : an organism living in, with, or on another organism in parasitism 3 : something that resembles a biological parasite in dependence on something else for existence or support without making a useful or adequate return
— par·a·sit·ic  \ˌper-ə-ˈsi-tik, ˌpa-rə-\ also par·a·sit·i·cal  \-ti-kəl\ adjective
— par·a·sit·i·cal·ly  \-ti-k(ə-)lē\ adverb
synonyms parasite , sycophant , toady , leech , sponge mean a usually obsequious flatterer or self-seeker. parasite applies to one who clings to a person of wealth, power, or influence or is useless to society <a jet-setter with an entourage of parasites>. sycophant adds to this a strong suggestion of fawning, flattery, or adulation <a powerful prince surrounded by sycophants>. toady emphasizes the servility and snobbery of the self-seeker <cultivated leaders of society and became their toady>. leech stresses persistence in clinging to or bleeding another for one’s own advantage <a leech living off his family and friends>. sponge stresses the parasitic laziness, dependence, and opportunism of the cadger <a shiftless sponge, always looking for a handout>.