Viruses are distinct from all other living species, whether eukaryotes or prokaryotes, in three ways: (1) the environment in which they grow and multiply, (2) the nature of their genome, and (3) the manner of their replication. To begin with, they can only function and multiply inside another live organism, which could be a prokaryotic or eukaryotic cell depending on the virus. Viruses are obligate parasites that are acellular and biologically inert outside of the host cell. Viruses are a diverse group of microorganisms with varying sizes, morphologies, and chemical compositions. Virion sizes range from 20 nanometres (parvovirus) to 300 nanometres (poxvirus). Some viruses are spherical (round), whereas others are filamentous or pleomorphic. Naked (non-enclosed) viruses usually have definite shapes and sizes, although some enveloped viruses (especially those with helical nucleocapsids) are very pleomorphic (e.g., orthomyxoviruses), with morphologies ranging from spherical to filamentous.