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 non-life. 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 wraith-like 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.
Less an organism than a jumbled collection of biochemical shards,the virus eventually yielded Wendell M. Stanley, the leader of the research team that exposed it, a Nobel Prize in chemistry rather than biology. The discovery also set off an intense scientific and philosophical debate that still rages: What exactly is a virus? Can it properly be described as alive? "'Life' and 'living' are words that the scientist has borrowed from the plain man," the British virologist Norman Pirie wrote at the time. "Now, however, systems are being discovered and studied which are neither obviously living nor obviously dead, and it is necessary to define these words or else give up using them and coin others."
Seventy years later, the challenge continues to haunt science. So "other" are viruses that we're still trying to corral them with new metaphors: microzombies, pirates of the cell, submicroscopic hijackers. But even the more restrained characterizations betray a long-standing prejudice. Most biologists typically recognize three official branches of life: the eukaryotes, which are organisms whose cells have a nucleus; bacteria, the single-celled organisms that may or may not possess a nucleus; and archaea, an ancient line of microbes without nuclei that may make up as much as a third of all life on Earth (See"Will the Methane Bubble Burst?" Discover, March 2004). Viruses, being dependent on these organisms to host them, are viewed as evolutionary latecomers: genomic scraps that fell out onto the floor back when life was assembling itself into more complex arrangements.
