What is the definition of a parasite?

A parasite is defined as an organism that benefits at the host's expense. This relationship typically involves the parasite living on or in the host and deriving nutrients or other benefits while harming the host in the process. Parasites can cause various health issues and may affect the well-being of the host, manifesting in various forms such as worms, protozoa, or ectoparasites like fleas.

The other definitions provided do not capture the essence of what a parasite is. An organism that is beneficial to its host represents a mutualistic relationship, where both parties gain advantages, which is the opposite of a parasitic relationship. Describing a microorganism that causes food spoilage does not specifically relate to the harmful benefits at the host's expense, as spoilage does not involve a host in the traditional sense used for parasites. Lastly, a disease-causing bacterium found in food suggests a pathogenic relationship but does not specifically relate to the parasitic nature of deriving benefits from a host organism. Hence, the clear definition points to option C as the correct characterization of a parasite.