When a sub-therapeutic dose is prescribed, or a patient chooses to use less of their prescribed antibiotic, bacteria are given the opportunity to adapt to the treatment. At lower doses, or when a course of antibiotics is not completed, certain strains of the bacteria develop drug-resistant strains through the process of natural selection.[13] This is due to the random genetic mutations that are constantly occurring in many forms of living organisms, bacteria and humans included. Natural selection supports the persistence of strains of bacteria that have developed a certain mutation that allows them to survive.