Viruses (from Latin virus, "poison" or "toxin") are small infectious agents, most of them 20-300nm in diameter. Viruses are microorganisms that cause various diseases, are considered obligate intracellular parasites, are acellular organisms, have no metabolism of their own, and have no organelles. When they parasitize a cell, they induce the production of viral genetic material and proteins, controlling the cellular metabolism. Viruses are acellular organisms, and their structure is composed of ...