A parasite is defined as an organism that lives on or within another living organism from which it derives benefit e.g. nutritional gain, and usually results in damage to the host organism in the form of disease. Parasites can include some protists e.g. the malaria parasite or plasmodium [which is a protozoon [plural protozoa]], but can also be larger organisms such as tapeworms [Taenia sagittarium] living within the human intestine and "stealing our food" or Onchocerca volvulus, a parasite that exists in the human eye. A parasite that can live in a living or dead host is called a facultative parasite, while one that requires its host to be alive is an obligative parasite.
Hope this helps!