Viruses are sub cellular, ultramicroscopic infectious agents of 20 nm-300 nm size range Many of them cause diseases like polio, dengue, small pox, measles, rabies, common cold etc., in animals and mosaic, dwarfing, curling etc. in plants. They are obligate intracellular parasites. In other words, they multiply only inside living cells. They consist of a protein coat or capsid surrounding a nucleic acid core of either DNA or RNA.Viruses were discovered in 1892 by a Russian biologist Iwanowsky, as'filter-passing agents' though they were seen much later, with the advent of electron microscope.
They share certain characteristics of both living and non-living things. So it will be useful to examine the properties of viruses. They are considered living because they can reproduce even though only when they are inside host cells, using the host cell machinery. On the other hand, they have some nonliving properties as well. When they are outside the host cell they can also undergo crystallisation like any other chemical substance. They do not have metabolism and so are metabolically inert. They cannot take up metabolic substances from the environment and lack the biochemical machinery like ribosomes or ATP generating systems to synthesize nucleic acid and proteins. They do not have their own enzyme system. They also do not exhibit growth or respond to stimuli.