A parasite is an organism that lives off or in another organism (its host), obtaining nourishment and protection while offering no benefit in return. Parasites are usually very harmful to its host, causing diseases to it. Human beings can be parasitic in nature.
Friends, family or any person who uses others to their own advantage without contributing anything positive in exchange is a parasite.
In the same way that parasitic organisms are harmful to its host, so is a human parasite. This sort of people stick around you only because of what they can gain from you. They are there in your life to rip you off and milk you dry. They are only around when everything seems to be going well for you.
All in all, staying away from any unfruitful, unproductive relationship; once noticed, would be a wise decision.