Here's why:

* Parasitism is a relationship where one organism (the parasite) benefits at the expense of another organism (the host).

* Viruses are obligate intracellular parasites, meaning they cannot replicate on their own and need a host cell to reproduce.

* When a virus enters a host cell, it uses the host cell's resources and machinery to make copies of itself, often causing harm to the host cell in the process.

So, in short, when a virus uses another organism for its own benefit, it is exhibiting parasitic behavior.