Carl Zimmer previews his next book:

A flu virus has just ten genes, for example, but a number of giant viruses have well over a thousand. Giant viruses even get infected by viruses of their own. For years, researchers have been finding that the diversity of genes in viruses is tremendous. It turns out that giant viruses are particularly bizarre, genetically speaking. The genes are so different, the scientists argue, that giant viruses represent a fourth domain of life.