Which category includes organisms that live on or in a host and cause harm?

The category that includes organisms that live on or in a host and cause harm is parasites. Parasites derive some or all of their nutritional requirements from their host, which can lead to various detrimental effects on the host's health. They often have specialized adaptations that allow them to live in or on the host organism, leading to a symbiotic relationship that is harmful rather than beneficial.

In contrast, while pathogens can also be harmful, they encompass a broader category that includes various types of disease-causing organisms, such as bacteria, viruses, and fungi, but not all pathogens live on or in a host as parasites do. Fungi and viruses have distinct roles and characteristics that do not align with the definition of organisms that specifically establish themselves within a host to the degree of harm typically associated with parasites. Thus, "parasites" is the most accurate term to describe organisms that exploit their hosts to their own benefit while inflicting damage.