Answer:
A parasite
Explanation:
Parasitism is a form of symbiosis where one organism lives on/in another organism, as in the question, and gets a benefit from that organism, while the other organism is harmed. This is denoted by +/-, which means that one is positively impacted and one is negatively impacted, as opposed to commensalism, which is +/0, or mutualism, which is +/+.
An example is tapeworms, which are flatworm parasites that live in the human intestine; they benefit while the host suffers.
