Which type of relationship involves a parasite benefiting at the expense of its host?

Parasitism is a relationship where one organism, the parasite, benefits by deriving nutrients at the expense of another organism, the host. In this relationship, the parasite often lives on or within the host, leading to various impacts on the host's health, which can range from minor inconvenience to severe harm or even death. This dynamic highlights a clear distinction between the two participants, as only the parasite receives a net benefit, while the host experiences negative effects.

In contrast, mutualism involves a relationship where both parties benefit, and commensalism features one organism benefiting while the other is neither helped nor harmed. Symbiosis is a broader term that encompasses all types of close ecological relationships, including mutualism, commensalism, and parasitism. Only parasitism clearly indicates the situation where the host suffers for the parasite's gain.