What is the definition of a parasite?

A parasite is defined as an organism that feeds on a living host. This relationship is characterized by the parasite deriving benefits at the expense of the host, often leading to harm or adverse effects on the host's health. Parasites can be classified into various types, including protozoa, helminths, and ectoparasites like fleas and ticks, all of which rely on a host for nourishment and survival.

In contrast, the other options describe different biological entities. For instance, living organisms that do not require a host for survival are considered free-living organisms, while bacteria are unicellular microorganisms, not specifically classified as parasites unless they engage in a parasitic relationship. Additionally, a non-living entity that causes disease pertains more to viruses, prions, or toxins, which do not fit the definition of a parasite since they do not live on or feed from a host organism. Thus, understanding the correct definition of a parasite is crucial for differentiating it from other biological interactions.