Many people are familiar with the antibiotic penicillin, which was discovered during the 1920s by Alexander Fleming. Antibiotics come in pill form, as syrups, as salves and ointments, and are also given by injection. Antibiotics kill both good and bad bacteria. Each time you take an antibiotic, bacteria that are sensitive to the antibiotic are killed. Other bacteria are not killed as easily and may grow and multiply - they are "resistant" to the antibiotic. Over many years, some bacteria have become resistant to antibiotics commonly used to treat infections caused by these bacteria. "Antibiotic resistance" describes this process. These bacteria have slowly changed over time and are no longer sensitive to some antibiotics. In other words, some antibiotics no longer work very well against some infections caused by resistant bacteria.