Which organisms live on or in a host and rely on it for nourishment?

• A. Parasites
• B. Virus
• C. Bacteria
• D. Fungi

Answer: (A)

Parasites are organisms that live on or inside another organism, the host, and depend on the host for nutrients and energy. This relationship, called parasitism, means the parasite derives nourishment from the host while often harming it. Examples include tapeworms in the gut or lice on the skin. Viruses don’t feed on the host in the same way; they need host cells to replicate and use the host’s machinery rather than drawing nourishment from the host as a resource they consume themselves. Bacteria and fungi can be free-living or opportunistic parasites, but the defining idea here is the reliance on the host for sustenance, which is precisely what parasites do.