VIRUSES ARE SUBMICROSCOPIC organisms that can be seen only with an electron microscope. Their size is measured in nanometers; a nanometer is one billionth of a meter.

As small as they are, viruses are responsible for untold damage and suffering. They can infect many types of plants and animals, producing a wide range of illnesses and even death. They do this by entering a living cell and taking over the cell in order to replicate themselves. In fact, viruses cannot live on their own.

They require a host animal or plant in which they can replicate so they can survive, and they must get inside a living cell to begin this process. "A virus relies entirely upon the host cell's ability to create the energy necessary to perform all of the manufacturing processes. Viruses do not come with...