A tiny realm, a gigantic reach

The Invisible Enemy

A Natural History of Viruses

Dorothy H. Crawford

Oxford University Press; 276 pages; $14.95 paperback

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Although they’ve been around since the dawn of life on Earth, viruses weren’t seen until 1938, thanks to the invention of the electron microscope.

Even so, they’ve managed to make their presence felt throughout human history by causing devastating epidemics, just as the outbreak of severe acute respiratory syndrome is threatening to do now.

Dorothy H. Crawford’s “The Invisible Enemy” provides an intriguing tour of this tiny realm that most people don’t think about unless it grabs the world’s headlines or causes them to wake up one morning with a sore throat that won’t go away.

What exactly are viruses? They’re certainly smaller than bacteria (“if a virus was the height of a man,” writes virologist Crawford, “an average-sized bacterium would be as high as the Statue of Liberty”). And while bacteria can make it on their own, equipped with the stuff that makes cells autonomous, viruses are just bits and particles of life -- “little more than a piece of genetic material protected by a protein coat” -- and it’s this inability to survive on their own that makes them so nefarious.

Crawford shows how these clever parasites -- from the common cold to Ebola and HIV -- “enter” a cell by mimicking various chemical receptors on a cell’s surface (imagine keyholes on a door) with a “skeleton key.” The body’s immune system handily wipes out some invaders, like the flu, while others, such as HIV, wage a long war of attrition against the body’s killer T-cells and other defenses.

“The Invisible Enemy” is a frustrating read for suggesting how little we know about viruses -- millions of years of existence but barely a century of recognition. And yet, Crawford is upbeat, describing successful campaigns to eradicate polio and smallpox, the development of antiviral medications and breakthrough discoveries like that of the Epstein-Barr virus in 1964, which provided a suspected link between viruses and certain forms of cancer.

Crawford also looks at that strange triangle -- human, fowl, pig -- in which flu viruses move, mutate and spread, giving rise to epidemics and fueling fears that a still unknown killer virus is lurking in some corner of the world. She’s optimistic about the future, envisioning a brave new world in which engineered viruses will fight such illnesses as cancer. Yet she’s also guarded, allowing a question straight from a sci-fi movie trailer to linger over the book’s final pages: “Who will be the final victor -- man or microbe?”