As scientists study the genes of more and more strains of E. coli, they're finding that foreign DNA has been steadily pouring into the genome. Not only is E. coli mutating within itself, it's also claiming new genes from elsewhere. A major source of this input is viruses. As Zimmer notes, "Viruses are quickly losing their reputation as insignificant parasites." Viruses, we now know, pick up genes from one host and plug them like cassette tapes into the genome of a new host. This sort of gene-leapfrogging is called horizontal gene transfer, and it's not limited to bacteria and viruses. We've already identified around 100,000 viruses in the human genome, and the vestiges of 150,000 more. What findings like this, and writers as capable as Zimmer, force us to ask is: What does it mean to be a human being? Are the barriers between species really as distinct and inviolable as we think they are? If human beings were nothing at all like bacteria, why would pharmaceutical companies be able to successfully plug human genes into microbes like E. coli?