Viruses do not have cells, so we can say that viruses are acellular organisms, the structural and functional unit of living things. This feature contradicts the Cell Theory, which says that all living things are formed by cells. Therefore, because they do not have cells, many claim that viruses are not living things. Viruses are only able to reproduce within a host cell. For this reason we say that they are obligate intracellular parasites.
