What defines a virus?

A virus is defined as acellular, meaning it does not consist of cells and is not classified as a living organism in the traditional sense. Additionally, viruses require a host organism in order to replicate. They lack the machinery necessary for metabolic processes and reproduction, relying on the host's cellular mechanisms to produce new virus particles (virions). This dependency on a host is a key characteristic that distinguishes viruses from more complex life forms, which can reproduce independently and carry out all necessary life functions on their own. The other options describe characteristics of living cells or organisms, which do not apply to viruses.