Take penicillin, for example, which was an effective treatment up until the 1960s. At that time, scientists observed that a few strains of gonorrhea had begun manufacturing an enzyme called penicillinase, which renders penicillin useless. Then, in the 1980s, strains of gonorrhea that did not make penicillinase also became resistant to penicillin -- by other means, such as chromosomal mutations.
So, in 1986, the CDC began monitoring antibiotic resistance in gonorrhea.
By 2010, almost 30% of the samples analyzed were resistant to the following antibiotics: penicillin, tetracycline, and ciprofloxacin (or some combo thereof), which means whole swaths of medicine that used to clear up the clap just fine now simply… don't work. At all.
Why has this happened? For a few reasons: one, as in the case of penicillin, is that antibiotics are often over-prescribed, and the more often bacteria are exposed to antibiotics, the more likely it is that some will develop resistance and continue spreading. If you take antibiotics when you have the sniffles, you're part of this problem -- but so is the doctor who gave them to you.