Antibiotics are designed to wipe out infection-causing bacteria, but even a single course can leave a lasting mark on the gut ...

A common type of penicillin used to treat syphilis is in short supply in the U.S. as cases of the sexually transmitted disease continue to rise. Penicillin G benzathine, an antibiotic sold in the U.S.

These days, we don't think much about being able to access a course of antibiotics to head off an infection. But that wasn't always the case—antibiotics have been available for less than a century.