For decades, descriptions of viruses have straddled life and nonlife, a divide that usually isn’t difficult to navigate. Their hallmark characteristics, namely their small size, tiny genomes and parasitic dependence on cellular hosts for replication, set them apart from all other living things despite their animation. But that story has gotten far more puzzling — particularly since the discovery of the first giant virus in 2003, which was so large that researchers initially thought it was a bacterium.
Several families of giant viruses are now known, and some of those giants have more than 1,000 genes; one has a whopping 2,500. (By comparison, some small viruses have only four genes.)