Viruses do not make their own food but depend on other living cells,which is why they are called parasites.
Best Answer: A parasite derives benefit( for protection, nutrition, reproduction etc.) from the host and in this process harm is caused to the host.
A virus is obliged to enter a host cell in order to duplicate its components. A virus does not have its own ribosomes and other materials by which it could manufacture its own molecules.Upon the entry of the cell, it usurps cellular machinery and uses its ribosomes and molecular units for the manufacture of its structure and genetic material. The cell becomes exhausted and depleted and finally dies. Hence virus is an obligatory cellular parasite.