Carl Zimmer is a science writer whose byline appears frequently in the New York Times. His books include A Planet of Viruses; Microcosm; Soul Made Flesh; and Parasite Rex. He has a busy blog and lectures at Yale.

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The frontier of biology these days is the genetics and ecology of bacteria, and the frontier of THAT is what's being learned about viruses. "The science of virology is still in its early, wild days," writes Carl Zimmer. "Scientists are discovering viruses faster than they can make sense of them." The Earth's atmosphere is determined in large part by ocean bacteria; every day viruses kill half of them. Every year in the oceans, viruses transfer a trillion trillion genes between host organisms. They evolve faster than anything else, and they are a major engine of the evolution of the rest of life. Our own bodies are made up of 10 trillion human cells, 100 trillion bacteria, and 4 trillion very busy viruses. Some of them kill us. Many of them help us. Some of them are us. Viral time is ancient and blindingly fast.

Science journalist Carl Zimmer is the author of A Planet of Viruses; the best introduction to the subject. His previous books include Parasite Rex and Microcosm.

What's time to a virus?

"Everything about viruses is extreme," Zimmer began.  The number of viruses on Earth is estimated to be 1 followed by 31 zeroes.  Small as they are, if you stacked them all up, the stack would reach 100 million light years.  They are the planet's most abundant organism by far.

They're fast.  We take decades to reproduce.  A flu virus can generate billions of itself in us within hours.  And they evolve 10,000 times faster than us, because they're creatively sloppy about making copies of their genomes, and they readily combine genes among varieties when jointly infecting a cell.  Each of us has four trillion viruses on board, in 1,500 all-too-fungible varieties.

Yet they can also be "time stealthy."  You may have a bout of childhood chickenpox that is over in days, but the viruses may hide in your nervous system and emerge decades later as shingles.  HIV spreads inexorably because of the lag of months or years between infection and visible symptoms.

The earliest record of a virus in human history is the smallpox marks you can see on the mummified face of Ramses V, who died in 1145 BCE. Viruses leave no fossils, but in a sense they ARE fossils, with the ancient gene sequences of retroviruses buried in the genomes of every creature they've infected over the ages.  About 8 percent of our genome---some 100,000 elements---comes from viruses, and some of those genes now work for us (enabling the mammalian placenta, for instance).  One French scientist revived from our genome a functioning 2-million-year-extinct virus just by deducing the original code from the current variety in that stretch of DNA.

For billions of years the planet's life consisted solely of bacteria and their viruses, the bacteriophages.  They became a planet force, and remain so today, determining the makeup of the atmosphere, among other things.  Every day half of all the bacteria in the oceans are killed by phages.  Some of the carbon from the bodies sinks to the bottom, some is freed up to fertilize other life.  Ocean viruses cart around and transmit genes for photosynthesis to previously incapable microbes---10 percent of oceanic photosynthesis happens that way.  If some day we have to geoengineer the atmosphere to manage climate change, we may want to employ the viruses that are already doing it.

Virology will be revolutionizing science for decades to come.  One body of investigation suggests that the so-called giant viruses may be a whole fourth domain of life (added to bacteria, archaea, and eukaryotes).  As the ultimate parasite, viruses were assumed to come along after life evolved, but they might an instrument of that evolution.  One hypothesis is that viruses took primordial RNA and generated DNA to better protect the genes.  They might have created life as we know it, a long time ago.