Which of the following best describes a parasite?

The correct answer describes a parasite as an organism that gains nourishment at the expense of another. Parasites rely on their host for sustenance and typically cause some level of harm to the host organism. This relationship is characterized by the parasite benefiting, while the host is negatively affected, often leading to complications such as disease, malnutrition, or even death of the host in extreme cases. Understanding this concept is essential for pest management and health sciences, as it underlines the need to control and manage parasitic relationships in ecosystems or agricultural settings. This definition distinguishes parasites from other types of organisms that may interact with hosts in different ways, such as predators or commensals, emphasizing the one-sided benefit that defines a parasitic relationship.

In contrast, the other options represent different biological interactions that do not fit the definition of a parasite. For instance, an organism that kills another is typically classified as a predator rather than a parasite, while a virus that infects other organisms may not always be dependent on a host in the same way as a traditional parasite. Additionally, while certain plants that grow on other plants could be considered parasites (like mistletoe), they do not encompass the broader definition of parasitism that emphasizes the harmful nutrient-gaining aspect.