. Infectious and parasitic microorganism much smaller than a bacterium, too small to be seen with a regular microscope or to be trapped even by ceramic filters. A virus is incapable of independent metabolic activity and replication, it must invade a live animal, human, or plant cell from which to derive the energy for survival. For replication, the virus provides only the genetic code and the invaded cell provides the raw material. The growth of viruses disrupts the invaded cell’s internal mechanism and it usually dies and releases the viruses in the bloodstream. The invaded body’s immune defenses react by producing an antiviral protein (interferon) which halts the spread of viruses to other cells.

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