I hate variegated plants. There, it's out. To me they look sickly, a little phoney and in most cases simply unattractive. It's the washed out yellow colour I dislike particularly. Reds I can stomach, sometimes.
Although we tend to blame variegation on viruses, mostly it's bad genes. For example the streaked leaves of shell ginger (Alpinia zurumbet), and presumably the single leaf of the Oyster Plant (Acanthus mollis) I've photographed here, are caused by cells within the plant mutating so that the production of the green pigment, chlorophyll, is stopped.
If these yellow cells are in what is called the meristematic area - where new tissues are produced - a streak or blotch ensues. Meanwhile neighbouring cells retain the original genetic makeup and keep producing green and more productive tissue.
Red coloured blotches are caused by an overproduction of anthocyanin, in this case masking the green of the chlorophyll. (The same thing happens in autumn leaves - yellow leaves are yellow because the chlorophyll has been withdrawn, red leaves are red because anthocyanin has been added.) The actual patterning of the variegation depends on where in the meristem the mutant cells arise.
Similarly I presume with viral infections - i.e. some are more obvious in their symptoms than others (and of course many a serious impact on agricultural crops). These days we can usually identify the virus responsible for a disease or a discolouring. For example the Bengal Tiger and Pretoria cultivars of Canna have been shown to be due to a Yellow Mottle Virus. And the stunted forms of Sacred Bamboo (Nandina domestica) with red, curled leaves are due to a virus called the Nandina Stem Pitting Virus.
In plants such as Sacred Bamboo it's not known how the virus gets from one individual to another but normally it is through sap-sucking insects or a piece of the plant itself. Where a virus-induced variegated plant is widely available in horticulture it is usually propagated by cutting or graft.
By the way, viruses are tiny packets of genetic material inside a protein protective case, too small to be seen in a normal optical microscope. They can only reproduce inside the cell of a living organism, although there is an ongoing debate about whether to call a virus living or non-living (or indeed whether the word 'life' is useful at all when there is a continuum of life-like chemical reactions on Earth).
There can also be some uncertainty about whether variegation in some species is caused by a random mutation or the arrival of a colour inducing virus. Although Tulip Breaking Virus was encouraged in 17th century Holland during 'Tulipmania', these days bulbs producing streaky flowers and sold under the name Rembrandt Tulips are generally free of viruses (or at least of viruses causing floral streaking).
I think it's fair to say that in most cases variegation is caused by mutated genes rather than viruses. No matter what the cause, plants with variegated leaves will be out competed by fully green plants because the yellow plant tissue doesn't have the chlorophyll required to trap the sun's energy and turn carbon dioxide and water into the sugars required for growth. And variegated flowers are likely to be less attractive to pollinating insects and animals that have evolved an attraction for a particular floral display. So you would expect to find few variegated plants in nature.
In (some) gardens, though, variegated plants thrive. They have seduced (some of) us into helping them survive and prosper. While I may not love them, I can turn a blind eye to a leaf or two.
Images: a mutant variegated leaf in a clump of Oyster Plant (Acanthus mollis) near A Gate in Royal Botanic Gardens Melbourne. Thanks to Dan Murphy, a colleague at the Royal Botanic Gardens, for suggesting this topic as well as Acacia chinchillensis.