Which term best describes an animal that exclusively lives within a host's body?

The term that best describes an animal that exclusively lives within a host's body is "endoparasite." Endoparasites are organisms that reside inside their host, often living in the tissues or organs of the host. This lifestyle is characteristic of many parasitic organisms, including some types of worms and protozoa, which derive nutrients and shelter from their host while potentially causing harm to it.

In contrast, ectoparasites live on the outside of their host's body, such as fleas or ticks, making them a different category of parasites. Macroparasites are larger parasites that can be either endoparasitic or ectoparasitic and typically have a longer life cycle, whereas commensals are organisms that live in a relationship with a host without harming or benefiting it. Thus, the correct choice emphasizes the internal relationship between the endoparasite and its host.