Virus is a biological entity which lives on another organism.

One word substitute is Parasite.

Dependant: a person who relies on another, especially a family member, for financial support.
Parasite: an organism that lives in or on an organism of another species (its host) and benefits by deriving nutrients at the other's expense.
Symbiotic: involving interaction between two different organisms living in close physical association.
Plebiscite: the direct vote of all the members of an electorate on an important public question such as a change in the constitution.