Cell theory states: all living things are composed of one or more cells; a cell is the smallest unit that still retains the properties of life; and new cells only come from cells that already exist. based on cell theory, why is a virus considered non-living? a) a virus does not have a nucleus. b) a virus does not contain genetic material. c) a virus is too small to be considered a cell. d) a virus cannot reproduce itself without a host cell.