endoparasite

This term is a technical biological classification used primarily in scientific, medical, and academic registers. It distinguishes organisms that reside within the internal tissues, organs, or blood of a host from ectoparasites, which live on the external surface of the skin or hair.

In a clinical context, the word carries a neutral, descriptive tone, though in general conversation it may evoke a sense of repulsion or illness due to the nature of internal infestation. It is rarely used in casual speech, where more general terms like parasite or worm are preferred.