Viruses are classified as non-living by most scientists. They do not breathe, eat, or do anything at all, really. When a virus attaches to a living cell, the attachment is a mechanical one, like a key fitting into a lock; the virus does not have to "do" anything to attach. Although a virus can take over the functioning of a living cell, that alone does not mean that it is alive. Many chemical toxins change the way a cell acts, too, and they are not alive. While the interaction between a virus and a host cell is complex, only the host cell is living. The virus is more like a robot.