 Furthermore, there is some debate over the definition of life, and what entities qualify as organisms; consequently the smallest known organism (microorganism) is debatable.Many biologists consider viruses to be non-living because they lack a cellular structure and cannot metabolize by themselves, requiring a host cell to replicate and synthesize new products.A minority of scientists hold that, because viruses do have genetic material and can employ the metabolism of their host, they can be considered organisms.