All told, some researchers have estimated the Earth may be home to 100 trillion species of viruses—most of which can be found at sea. Viruses evolve this diversity thanks to the peculiar way they multiply. When cells get infected, they create many new viruses, but they do so sloppily. The genes of the new viruses are rife with copying mistakes. While most of these mutations disable viruses, some of them give viruses an evolutionary edge, letting them infect hosts more effectively. If two kinds of viruses infect a cell at once, they may shuffle their genes together. Viruses can even end up with
...more
