Antibiotics kill most of the bacteria they meet, but a few survive, and those survivors pass on whatever allowed them to resist. Every course of treatment therefore leaves behind a population slightly harder to treat than the one before. This is not a failure of the drugs; it is the ordinary result of using them. What matters is the rate at which it happens, and the rate rises sharply when antibiotics are given for infections they cannot cure, such as colds and other viral illnesses.