Consider these facts from Carl Zimmer's book, A Planet of Viruses: If you put all the viruses in the ocean on a scale, they would equal the weight of 75 million blue whales. And if you lined up all those viruses end to end, "they would stretch out past the nearest 60 galaxies."

In 17th century England, Zimmer writes, cures for the rhinovirus, or the common cold, included a blend of gunpowder and eggs and fried cow dung and suet. Today, he says, doctors have little more to offer a person that's sick with a cold.

Zimmer is a contributing editor at Discover, an author of 12 books on science and a regular contributor to the New York Times. (He also has a tapeworm named after him.) We spoke with him recently about his latest book, in which he uncovers the bizarre world of viruses living in the soil, in caves underground and in our own bodies — while addressing the fundamental questions driving virologists. Why, for example, has it been so hard to find a penicillin for viruses? Which virus will become the next great pandemic? Will scientists ever develop a cure for the common cold? And should they?