How Resistance Happens:

Since their introduction in the 1940s, antibiotics have saved millions of lives. But as soon as bacteria had their first fight against the drugs, they began adapting ways to outsmart them.

How It Works:

You start life with a bunch of different kinds of bacteria on your skin and in your gastrointestinal tract, and most of them are sensitive to antibiotics. But a few are not, says Elizabeth Bancroft, M.D., a medical epidemiologist with the Los Angeles County Department of Public Health. As we kill off the vulnerable bugs with drugs, we begin to tilt the bacterial balance in the body: Any resistant germs now have a clear playing field. "They can start dividing and become the dominant strain in your body," she says. That's not the only trick bacteria have. They can also mutate and develop changes in their DNA that make them invulnerable to the antibiotics they've been exposed to before; and they can acquire resistance genes by swapping genetic material with other bacteria. Pearce found out just how adaptable and persistent bacteria can be when she noticed a new pimple on the back of Andy's thigh in January. Not only was the MRSA back, but the new infection was resistant to clindamycin, the medicine that had cured him before. "From the moment I realized what was going on, I was a wreck," she says.

Fortunately, the infection responded well to Bactrim, another antibiotic. But doctors warned the Pearces that Andy is probably colonized with resistant staph, meaning he permanently carries the bugs in his nose or on his skin, and they could trigger another infection at any time (an estimated 2.3 million Americans are MRSA carriers). "The fact that we're going to be dealing with this for the rest of his life is overwhelming," says Pearce.