Microbes: Can't live with them, can't live without them. We focus more on the first of these truths, as shown by the huge sums spent every year on preventing, treating, and researching infectious diseases. But the second is equally valid: Each human body contains at least 10 times as many microbes as human cells. The human gut alone contains about 1 kilogram of bacteria indispensable to digestion.


But sometimes microbes get out of hand, triggering everything from athlete's foot to influenza pandemics that kill tens of millions, and for millennia humans have sought ways to control them. People have tried everything from prayer, to natural biocides (Otzi, the 5,300-year-old hunter found frozen in an Alpine glacier in 1991, carried fungal oils to treat intestinal parasites), to cautery (scorching wounds with a hot iron).

The results were mixed at best. Then, in 1929, Alexander Fleming noticed a clear zone on an agar plate of staphylococci bacteria "contaminated" with Penicillium mold. Ten years later, Ernst Chain and Howard Florey found a way to isolate the active ingredient. Penicillin was used to treat infections in World War II and was commercialized immediately afterward. From this accidental beginning, the global anti-infective market has grown to nearly US$70 billion a year; antibiotics account for about half.


The chief downstream impacts of antibiotic use involve antibiotic resistance. Resistance is natural; microbes that produce antibiotics do so to stave off competition from other microbes, which in turn evolve to escape harm. This co-evolutionary dance is expanded and accelerated by the immense scale of human antibiotic use--and misuse.


Antibiotic resistance developed almost immediately after commercial production of antibiotics began in 1946. It has since become a major threat to the control of pathogens, nearly all of which are resistant to one or more standard antibiotics. Resistant pathogens have become common in institutional settings such as hospitals and nursing homes; according to the U.S. Centers for Disease Control, about 70 percent of common hospital infections are resistant to at least one antibiotic. One of the most common institutional microbes, methicillin-resistant Staphylococcus aureus, kills more people in the United States each...

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