Viral diseases

Viruses cause disease across the living world - in plants (e.g. tobacco mosaic, cauliflower mosaic, rice tungro disease), in domestic animals (e.g. foot-and-mouth disease of cattle, rabies of dogs), and in humans (e.g. the common cold, hepatitis B, SARS, AIDS, mumps, polio, chickenpox and measles). Beyond the 'true' viruses covered so far, a few related sub-viral agents are also worth knowing: a virion is simply a complete, intact, infective virus particle that cannot replicate outside a host cell. A viroid (discovered by T.O. Diener, 1971) is a naked circular single-stranded RNA molecule with no protein capsid at all, low molecular weight, that causes diseases such as citrus exocortis and potato spindle tuber disease. A virusoid (discovered by J.W. Randles, 1981) is similar to a viroid - a small circular RNA - but it never occurs on its own, always being packaged together with a larger viral RNA. Prions (discovered by Stanley B. Prusiner, 1982) are infectious agents made purely of misfolded protein, with no nucleic acid at all, responsible for fatal degenerative nervous-system diseases such as Creutzfeldt-Jakob Disease and 'mad cow disease' (Bovine Spongiform Encephalopathy) in humans and animals, and scrapie in sheep. Finally, viruses also attack microbes: those infecting cya …