Relman is a leader in rethinking our relationship to bacteria, which for most of the last century was dominated by the paradigm of Total Warfare. “It’s awful the way we treat our microbes,” he says, not intending a joke; “people still think the only good microbe is a dead one.” We try to kill them off with antibiotics and hand sanitizers. But bacteria never surrender; if there were one salmonella left in the world, doubling every 30 minutes, it would take less than a week to give everyone alive diarrhea. In the early years of antibiotics, doctors dreamed of eliminating infectious disease. Instead, a new paper in The Journal of the American Medical Association reports on the prevalence of Methicillinresistant Staphylococcus aureus (MRSA), which was responsible for almost 19,000 deaths in the United States in 2005—about twice as many as previously thought, and more than AIDS. Elizabeth Bancroft, a leading epidemiologist, called this finding “astounding.”

As antibiotics lose their effectiveness, researchers are returning to an idea that dates back to Pasteur, that the body’s natural microbial flora aren’t just an incidental fact of our biology, but crucial components of our health, intimate companions on an evolutionary journey that began millions of years ago.