What are organisms called that grow and feed on or in other organisms without contributing to the survival of the host?

Organisms that grow and feed on or in other organisms while not contributing to the host's survival are known as parasites. These organisms rely on a host for nutrients and often cause harm in the process, which is the defining characteristic of parasitism. Parasites can take many forms, including protozoa, worms, and insects, and they often have specialized adaptations that allow them to thrive at the expense of their hosts.

In contrast, while bacteria and fungi can also be associated with living organisms, not all of them are harmful. Some bacteria are beneficial and essential for processes like digestion, while certain fungi can also play roles in ecosystems that do not harm their hosts. Viruses, on the other hand, are different in that they require a host to replicate and are not classified as living organisms; they invade host cells and often disrupt normal cellular functions, which may lead to disease. Therefore, the term "parasite" is specifically appropriate for organisms that have a detrimental effect on their hosts without providing any benefit.