Dr. Alastair D. Hay, who teaches medical students at the University of Bristol in England and also treats patients, says that until recently, even he may occasionally have succumbed to the pressure to hand over a prescription.

"As a personal policy, I don't get into heated arguments with my patients," Dr. Hay said.

And giving the standard lecture about how antibiotics will not stop a virus but may contribute to the growing, worldwide problem of drug resistance rarely convinces sick people that they don't need the drugs. "Unless you can tell them that there's an immediate downside for them personally," Dr. Hay said, "the message just doesn't sink in."

Now, though, Dr. Hay can quote direct evidence of a downside. An increasing number of studies, including his own work, suggest that even a properly prescribed antibiotic can foster the growth of one or more strains of antibiotic-resistant bacteria for at least two to six months inside the person taking the pills.

"Carrying" a microbe inside you that is resistant to drugs also means that, during that time, you're likely to "share" the resistant bug with family, co-workers and others in your path.

That particular strain may not make you sick. But if you find yourself one day immune-suppressed after chemotherapy, cut open by a car accident or surgery or especially vulnerable to bacterial pneumonia after a bad flu, those resistant strains of bacteria living inside you increase the odds that any infection will be hard - or even impossible - to beat.