The nature of viruses made them impossible to detect for many years even after bacteria had been discovered and studied. Not only are viruses too small to be seen a light microscope, they also cannot be detected through their biological activity, except as it occurs in conjunction with other organisms. In fact viruses show no traces of biological activity by very themselves. Unlike bacteria, they are not living agents in the strictest sense viruses are simple pieces of organic material composed only of nucleic acid, either DNA or RNA, enclosed in a coat of protein made up of simple structural units. (Some viruses also contain carbohydrates and lipids.) They are parasites, requiring human, animal, o plant cells to live. The virus or replicates by attaching to a cell and injecting its nucleic acid.' once inside the cell, the DNA RNA that contains the virus' genetic information takes over the cell's biological machinery, and the cell begins to manufacture viral proteins rather than its own.