单项选择题

The term "virus" is derived from the Latin word for poison, or slime. It was originally applied to the noxious stench emanating from swamps that was thought to cause a variety of diseases in the centuries before microbes were discovered and specifically linked to illness. But it was not until almost the end of the nineteenth century that a true virus was proven to be the cause of a disease.
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 with 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 themselves. Unlike bacteria, they are not living agents in the strictest sense. Viruses are very simple pieces of organic material composed only nucleic acid, either DNA or RNA, enclosed in a coat of, protein made up of simple structural units. They are parasites, requiring human, animal or plant cells to live. The virus replicates by attaching to a cell and injecting its nucleic acid; once inside the cell, the DNA or RNA that contains the virus’ genetic information takes over the cell’s biological machinery, and the cell begins to manufacture viral protein rather than its own.

Before microbes were discovered it was believed that some diseases were caused by().

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