What is an orginasim that feeds off of a living host?

Parasites that live in the body are called endoparasites. An example of an endoparasite is a hookworm that lives in the hosts gut. 

Surrounding every nucleus is a double-layered membrane called the nuclear membrane or nuclear envelope. This membrane separates the nucleoplasm, or fluid inside the nucleus, from the cytoplasm, or fluid outside the nucleus.

Cotton was the very first