A parasite is an organism that lives in another organism, called the host, and often harms it. It is dependent on its host for survival - it has to be in the host to live, grow and multiply. A parasite cannot live independently. Although a parasite rarely kills the host.
A parasitic disease is an infectious disease caused or transmitted by aparasite. Many parasites do not cause diseases. Parasitic diseases can affect practically all living organisms, including plants and mammals. The study of parasitic diseases is called parasitology.
hope it help u