An ultramicroscopic infectious agent that can only replicate within the cells of living hosts, mainly bacteria, plants, and animals. Viruses have a very basic structure, consisting of an RNA or DNA core surrounded by a protein coat. More complex viruses also have an envelope surrounding the core and protein coat. Viruses have a wide range of effects on their hosts, from nearly harmless (such as toxoplasmosis in cats) to fatal (such as Ebola in humans).

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