With deadly new viruses emerging these days in Saudi Arabia and China, it can be hard to imagine that viruses can be good for anything. It's easy to forget that we are home to trillions--perhaps quadrillions--of viruses on our healthiest days. And, according to a team of California scientists, those viruses are our symbiotic partners, creating a second immune system. These viruses serve as a defensive front-line, keeping bacteria from invading our gut lining and causing deadly infections.

The viruses in question are far less familiar than, say, influenza viruses or Ebola viruses. They are known as bacteriophages, which means "eater of bacteria." And yet bacteriophages (or ...