Few things on Earth are spookier than viruses. The very name virus, from the Latin word for "poisonous slime," speaks to our lowly regard for them. Their anatomy is equally dubious: loose, tiny envelopes of molecules—protein-coated DNA or RNA—that inhabit some netherworld between life and nonlife. Viruses do not have cell membranes, as bacteria do; they are not even cells. They seem most lifelike only when they invade and co-opt the machinery of living cells in order to make more of themselves, often killing their hosts in the process. Their efficiency at doing so ranks them among the most fearsome killers: Ebola virus, HIV, smallpox, flu. Yet they go untouched by antibiotics, having nothing really biotic about them.
The existence of viruses was first surmised just over a century ago by Dutch botanist Martinus Beijerinck. He mashed up disease-riddled tobacco leaves and then passed the juicy pulp through a porcelain filter fine enough to trap everything down to the tiniest bacteria. When even that filtered fluid infected other plants, a world still acclimating to Louis Pasteur's germ theory now had an even tinier class of pathogens to contemplate. Here were entities so wraithlike that they remained unseen until 1935, when scientists armed with the newly invented electron microscope managed to take a picture of the "poison" lurking in Beijerinck's slime, today known as tobacco mosaic virus.
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A pretty long read but it was very interesting. Also talks about the largest known virus Mimi with over a 1,000 genes and how it maybe a missing link to the pathogenic past.
http://www.discover.com/issues/mar-06/cover/
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