A parasite is defined as "An organism that grows, feeds, and is sheltered on or in a different organism while contributing nothing to the survival of its host."  Most definitions also mention that the parasite, usually the smaller of the two, often benefits while the host is harmed. 

Human beings are, quite literally, a parasite to planet earth.  They use it, burn it, dig it, crush it, and destroy it.  Then they scatter all their waste products (created and bodily) on the ground and in the ocean, further harming themselves and other creatures attempting to live symbiotically on the planet.  Every part of the planet is made worse by human interaction.  Humans make use of the earth for their own well-being at the expense of the earth's well-being, thus perfectly fitting the definition of a parasite. #psychology