In some situations, antibiotics are lifesavers. In others, however, they do more harm than good. For instance, when antibiotics are used too much or for the wrong illnesses, the drugs only end up killing helpful microbes and spawning drug-resistant superbugs. To figure out the proper times to use antibiotics, doctors need to carefully weigh the risks and benefits of each situation. But, sadly, that calculation is extremely tricky—if not impossible—because scientists still aren’t sure what all of the risks are.