Viruses are very small biological entities that lack cellular organization i.e. plasma membrane and metabolic machinery. They posses only the genetic material (DNA or RNA) of their own. Viruses cannot replicate on their own. They require a host cell. They utilize the host cell machinery for their replication.
Viruses are acellular entities and can only perpetuate inside a living host. Hence, they are called as obligatory intracellular parasites.