Viruses are not included in any of the kingdoms. They are visible only under an electron microscope, and do not have a cellular structure but consist of nucleic acid surrounded by an outer protein coat (a capsid). They do not have the cytoplasm, organelles and internal membranes found in the cells of living organisms. They cannot grow, move or reproduce without access to the cells of a host cell so they are not included in the classification of living things. Viruses survive by invading the cells of other organisms, modifying their behaviour and often causing disease e.g. arabis mosaic, chrysanthemum stunt, cucumber mosaic, leaf mosaic, plum pox, reversion, tomato mosaic and tulip break.
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